I enabled KMS debug for mutter, and captured some additional info.

Notabley the monitors on CRTC 167 and 236 are initially shown active: 0 and CRTC 98 (laptop screen) is shown active: 1

Startup Event: 
Jun 17 11:21:03 imac-X1-Yoga gnome-shell[2977]: Running GNOME Shell (using mutter 46.0) as a Wayland display server
Jun 17 11:21:03 imac-X1-Yoga gnome-shell[2977]: Made thread 'KMS thread' realtime scheduled
Jun 17 11:21:04 imac-X1-Yoga gnome-shell[2977]: Device '/dev/dri/card1' prefers shadow buffer
Jun 17 11:21:04 imac-X1-Yoga gnome-shell[2977]: KMS: Read CRTC 98 state: active: 1, mode: 3840x2400, changed: yes
Jun 17 11:21:04 imac-X1-Yoga gnome-shell[2977]: KMS: Read CRTC 167 state: active: 0, mode: (nil), changed: no
Jun 17 11:21:04 imac-X1-Yoga gnome-shell[2977]: KMS: Read CRTC 236 state: active: 0, mode: (nil), changed: no
Jun 17 11:21:04 imac-X1-Yoga gnome-shell[2977]: KMS: Read CRTC 305 state: active: 0, mode: (nil), changed: no
Jun 17 11:21:04 imac-X1-Yoga gnome-shell[2977]: Added device '/dev/dri/card1' (i915) using non-atomic mode setting.

Both monitors and the panel have their color profiles setup
Jun 17 11:21:05 imac-X1-Yoga gnome-shell[2977]: COLOR: Created new color device 'xrandr-LG Electronics-LG HDR 4K-0x00064f1f' for monitor DP-5
Jun 17 11:21:05 imac-X1-Yoga gnome-shell[2977]: COLOR: Created new color device 'xrandr-LG Electronics-LG HDR 4K-0x00064f20' for monitor DP-9
Jun 17 11:21:05 imac-X1-Yoga gnome-shell[2977]: COLOR: Created new color device 'xrandr-California Institute of Technology-0x1403-0x00000000' for monitor eDP-1

The mouse appears on both monitors (even though one, CRTC 167 is off from a user perspective)
Jun 17 11:21:05 imac-X1-Yoga gnome-shell[2977]: KMS: Realizing HW cursor for cursor sprite for CRTC 167 (/dev/dri/card1)
Jun 17 11:21:05 imac-X1-Yoga gnome-shell[2977]: KMS: Realizing HW cursor for cursor sprite for CRTC 236 (/dev/dri/card1)

Things look fine with mode setting
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Setting plane 169 (/dev/dri/card1) rotation to 1
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Setting mode of CRTC 236 (/dev/dri/card1) to 3840x2160
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Setting plane 100 (/dev/dri/card1) rotation to 1
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Setting mode of CRTC 167 (/dev/dri/card1) to 3840x2160

GDM fires up, and we see the first mode set error from mutter
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: Registering session with GDM
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: Swap buffers: 1 frames pending (double-buffering)
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: Assigning buffer to primary plane update on CRTC (167) with src rect 0.000000,0.000000 3840.000000x2160.000000 and dst rect 0,0 38>
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: Posting primary plane composite update for CRTC 167 (/dev/dri/card1)
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Processing update
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Setting CRTC 167 (/dev/dri/card1) gamma, size: 256
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Moving HW cursor of CRTC 167 (/dev/dri/card1) to (3836, 2155)
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Page flipping CRTC 167 (/dev/dri/card1) with 369, data: 0x718f14052b60
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Scheduling page flip retry on CRTC 167 (/dev/dri/card1)
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: Page flip failed: Page flip of 167 failed, and no mode set available

This seems to continue for both CRTC 167 and 236, however 236 does not return any errors, and instead there are posts related to page flip callbacks for CRTC 236


Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Handling page flip callback from /dev/dri/card1, data: 0x718f14051800, CRTC: 236

Lots more scheduled retrys for CRTC 167

Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Scheduling page flip retry on CRTC 167 (/dev/dri/card1)

For a brief period during GDM startup there seems to be successful callback from CRTC 167

Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: Posting non-primary plane update for CRTC 167 (/dev/dri/card1)
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Processing update
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Setting CRTC 167 (/dev/dri/card1) gamma, size: 256
Jun 17 11:21:06 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Handling symbolic page flip callback from /dev/dri/card1, data: 0x718f14051d40, CRTC: 167
Jun 17 11:21:16 imac-X1-Yoga gnome-shell[2977]: KMS: Swap buffers: 1 frames pending (double-buffering)
Jun 17 11:21:16 imac-X1-Yoga gnome-shell[2977]: KMS: Assigning buffer to primary plane update on CRTC (167) with src rect 0.000000,0.000000 3840.000000x2160.000000 and dst rect 0,0 38>
Jun 17 11:21:16 imac-X1-Yoga gnome-shell[2977]: KMS: Posting primary plane composite update for CRTC 167 (/dev/dri/card1)
Jun 17 11:21:16 imac-X1-Yoga gnome-shell[2977]: KMS: Swap buffers: 1 frames pending (double-buffering)
Jun 17 11:21:16 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Processing update
Jun 17 11:21:16 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Setting CRTC 167 (/dev/dri/card1) gamma, size: 256
Jun 17 11:21:16 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Page flipping CRTC 167 (/dev/dri/card1) with 387, data: 0x718f140032e0
Jun 17 11:21:16 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Scheduling page flip retry on CRTC 167 (/dev/dri/card1)
Jun 17 11:21:16 imac-X1-Yoga gnome-shell[2977]: KMS: Assigning buffer to primary plane update on CRTC (236) with src rect 0.000000,0.000000 3840.000000x2160.000000 and dst rect 0,0 38>
Jun 17 11:21:16 imac-X1-Yoga gnome-shell[2977]: KMS: Posting primary plane composite update for CRTC 236 (/dev/dri/card1)
Jun 17 11:21:16 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Processing update
Jun 17 11:21:16 imac-X1-Yoga gnome-shell[2977]: KMS: [simple] Page flipping CRTC 236 (/dev/dri/card1) with 389, data: 0x718f14051cc0
Jun 17 11:21:16 imac-X1-Yoga gnome-shell[2977]: Page flip failed: Page flip of 167 failed, and no mode set available

Then at one point later in the journal we see a mode set followed by the errors captured previously

Jun 17 11:21:23 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Setting mode of CRTC 167 (/dev/dri/card1) to 3840x2160
Jun 17 11:21:23 imac-X1-Yoga gnome-shell[3683]: Page flip failed: Failed to set mode 3840x2160 on CRTC 167: Invalid argument
Jun 17 11:21:23 imac-X1-Yoga gnome-shell[3683]: Page flip failed: Failed to set mode 3840x2160 on CRTC 167: Invalid argument
Jun 17 11:21:23 imac-X1-Yoga gnome-shell[3683]: Failed to post KMS update: Failed to set mode 3840x2160 on CRTC 167: Invalid argument

Eventually Wayland is fired up
Jun 17 11:21:24 imac-X1-Yoga gnome-shell[3683]: STARTUP: Window W1 has _NET_WM_USER_TIME of 34648

The sequence of page flipping errors continues, but there are periods when it seems normal for a few seconds

Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: Realizing HW cursor for cursor sprite for CRTC 236 (/dev/dri/card1)
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: Realizing HW cursor for cursor sprite for CRTC 167 (/dev/dri/card1)
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: Posting non-primary plane update for CRTC 236 (/dev/dri/card1)
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: Posting non-primary plane update for CRTC 167 (/dev/dri/card1)
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Processing update
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Setting HW cursor of CRTC 236 (/dev/dri/card1) to 47 (size: 256x256, hot: (6, 6))
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Moving HW cursor of CRTC 236 (/dev/dri/card1) to (3507, 2050)
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Processing update
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Setting CRTC 167 (/dev/dri/card1) gamma, size: 256
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Handling symbolic page flip callback from /dev/dri/card1, data: 0x787574053040, CRTC: 236
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Handling symbolic page flip callback from /dev/dri/card1, data: 0x7875740530c0, CRTC: 167
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: Posting non-primary plane update for CRTC 236 (/dev/dri/card1)
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Processing update
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Setting HW cursor of CRTC 236 (/dev/dri/card1) to 47 (size: 256x256, hot: (6, 6))
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Moving HW cursor of CRTC 236 (/dev/dri/card1) to (3546, 2026)
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Handling symbolic page flip callback from /dev/dri/card1, data: 0x787574053140, CRTC: 236
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: Posting non-primary plane update for CRTC 236 (/dev/dri/card1)
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: Posting non-primary plane update for CRTC 167 (/dev/dri/card1)
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Processing update
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Setting HW cursor of CRTC 236 (/dev/dri/card1) to 47 (size: 256x256, hot: (6, 6))
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Moving HW cursor of CRTC 236 (/dev/dri/card1) to (3588, 2003)
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Processing update
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Setting CRTC 167 (/dev/dri/card1) gamma, size: 256
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Setting HW cursor of CRTC 167 (/dev/dri/card1) to 92 (size: 256x256, hot: (6, 6))
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Moving HW cursor of CRTC 167 (/dev/dri/card1) to (-251, 2003)
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Handling symbolic page flip callback from /dev/dri/card1, data: 0x7875740531e0, CRTC: 236
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Handling symbolic page flip callback from /dev/dri/card1, data: 0x7875740532c0, CRTC: 167
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: Posting non-primary plane update for CRTC 236 (/dev/dri/card1)
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Processing update
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Setting HW cursor of CRTC 236 (/dev/dri/card1) to 47 (size: 256x256, hot: (6, 6))
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Moving HW cursor of CRTC 236 (/dev/dri/card1) to (3615, 1991)
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Handling symbolic page flip callback from /dev/dri/card1, data: 0x787574053100, CRTC: 236
Jun 17 11:21:25 imac-X1-Yoga gnome-shell[3683]: KMS: Posting non-primary plane update for CRTC 167 (/dev/dri/card1)

and again  

Jun 17 11:21:28 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Processing update
Jun 17 11:21:28 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Setting HW cursor of CRTC 236 (/dev/dri/card1) to 19 (size: 256x256, hot: (7, 7))
Jun 17 11:21:28 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Moving HW cursor of CRTC 236 (/dev/dri/card1) to (3764, -7)
Jun 17 11:21:28 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Processing update
Jun 17 11:21:28 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Setting CRTC 167 (/dev/dri/card1) gamma, size: 256
Jun 17 11:21:28 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Setting HW cursor of CRTC 167 (/dev/dri/card1) to 49 (size: 256x256, hot: (7, 7))
Jun 17 11:21:28 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Moving HW cursor of CRTC 167 (/dev/dri/card1) to (-75, -7)
Jun 17 11:21:28 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Handling symbolic page flip callback from /dev/dri/card1, data: 0x787574053950, CRTC: 236
Jun 17 11:21:28 imac-X1-Yoga gnome-shell[3683]: KMS: [simple] Handling symbolic page flip callback from /dev/dri/card1, data: 0x787574053790, CRTC: 167
Jun 17 11:21:28 imac-X1-Yoga gnome-shell[3683]: KMS: Posting non-primary plane update for CRTC 236 (/dev/dri/card1)
Jun 17 11:21:28 imac-X1-Yoga gnome-shell[3683]: KMS: Posting non-primary plane update for CRTC 167 (/dev/dri/card1)

eventually, settling into no more errors after about 45s after startup, presumably after I set the display to 60Hz manually with this last set below and no others in the hour that followed. 

16982:Jun 17 11:21:45 imac-X1-Yoga gnome-shell[3683]: Page flip failed: Failed to set mode 3840x2160 on CRTC 167: Invalid argument
16983:Jun 17 11:21:45 imac-X1-Yoga gnome-shell[3683]: Page flip failed: Failed to set mode 3840x2160 on CRTC 167: Invalid argument
16984:Jun 17 11:21:45 imac-X1-Yoga gnome-shell[3683]: Failed to post KMS update: Failed to set mode 3840x2160 on CRTC 167: Invalid argument

Hopefully this helps better understand what is going on. I put aside the full journal from this session with the drm_info