Adding this to *this* bug, despite the "Fix Released", because it exactly  matches the original post for this bug, whereas the others I've encountered and reported on are different:

Oct 14 20:02:45 fleetfoot gnome-shell[12750]: meta_monitor_manager_get_logical_monitor_from_number: assertion '(unsigned int) number < g_list_length (manager->logical_monitors)' failed
Oct 14 20:02:45 fleetfoot gnome-shell[12750]: meta_workspace_get_work_area_for_monitor: assertion 'logical_monitor != NULL' failed
Oct 14 20:02:45 fleetfoot kernel: [29646.218484] gnome-shell[12750]: segfault at 40 ip 00007f9354b11e60 sp 00007ffd0b949810 error 4 in libmutter-1.so.0.0.0[7f9354a6c000+141000]
Oct 14 20:02:46 fleetfoot gnome-shell[25511]: Couldn't register with accessibility bus: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.
Oct 14 20:02:46 fleetfoot gnome-shell[25511]: JS WARNING: [resource:///org/gnome/shell/ui/main.js 315]: reference to undefined property "MetaStage"
Oct 14 20:02:46 fleetfoot gnome-shell[25511]: JS WARNING: [resource:///org/gnome/shell/ui/layout.js 221]: reference to undefined property "MetaWindowGroup"
Oct 14 20:02:46 fleetfoot gnome-shell[25511]: JS WARNING: [resource:///org/gnome/shell/ui/osdMonitorLabeler.js 59]: reference to undefined property "MetaDBusDisplayConfigSkeleton"
Oct 14 20:02:46 fleetfoot gnome-shell[25511]: JS WARNING: [resource:///org/gnome/gjs/modules/tweener/tweener.js 540]: reference to undefined property "isSpecialProperty"
Oct 14 20:02:46 fleetfoot gnome-shell[25511]: JS WARNING: [resource:///org/gnome/shell/ui/slider.js 38]: reference to undefined property "CallyActor"
Oct 14 20:02:46 fleetfoot gnome-shell[25511]: meta_monitor_manager_get_logical_monitor_from_number: assertion '(unsigned int) number < g_list_length (manager->logical_monitors)' failed
Oct 14 20:02:46 fleetfoot gnome-shell[25511]: meta_workspace_get_work_area_for_monitor: assertion 'logical_monitor != NULL' failed
Oct 14 20:02:46 fleetfoot gnome-shell[25511]: Extension "<email address hidden>" had error: TypeError: this._monitor is undefined
Oct 14 20:02:46 fleetfoot gnome-shell[25511]: loading user theme: /usr/share//themes/Arc-Dark/gnome-shell/gnome-shell.css
Oct 14 20:02:46 fleetfoot gnome-shell[25511]: Some code accessed the property 'DBusMenu' on the module 'interfaces'. That property was defined with 'let' or 'const' inside the module. This was previously supported, but is not correct according to the ES6 standard. Any symbols to be exported from a module must be defined with 'var'. The property access will work as previously for the time being, but please fix your code anyway.
Oct 14 20:02:46 fleetfoot gnome-shell[25511]: Some code accessed the property 'WORKAROUND_RELOAD_TYPE_REGISTER' on the module 'util'. That property was defined with 'let' or 'const' inside the module. This was previously supported, but is not correct according to the ES6 standard. Any symbols to be exported from a module must be defined with 'var'. The property access will work as previously for the time being, but please fix your code anyway.
Oct 14 20:02:46 fleetfoot gnome-shell[25511]: Some code accessed the property 'StatusNotifierWatcher' on the module 'statusNotifierWatcher'. That property was defined with 'let' or 'const' inside the module. This was previously supported, but is not correct according to the ES6 standard. Any symbols to be exported from a module must be defined with 'var'. The property access will work as previously for the time being, but please fix your code anyway.
Oct 14 20:02:46 fleetfoot gnome-shell[25511]: Some code accessed the property 'StatusNotifierWatcher' on the module 'interfaces'. That property was defined with 'let' or 'const' inside the module. This was previously supported, but is not correct according to the ES6 standard. Any symbols to be exported from a module must be defined with 'var'. The property access will work as previously for the time being, but please fix your code anyway.
Oct 14 20:02:47 fleetfoot kernel: [29647.398006] gnome-shell[25511]: segfault at 40 ip 00007f67c43bce60 sp 00007ffd22dab610 error 4 in libmutter-1.so.0.0.0[7f67c4317000+141000]

Symptoms *observed*, again, were that I was logged out when I returned to the computer the following afternoon.

So this happened twice in quick succession. The above fragment is from a 'grep gnome-shell syslog.1', I can provide a fuller log if anyone's interested, but there was no /var/run/crash file generated from this event.

gnome-shell: 3.26.1-0ubuntu3
mutter & libmutter-1-0: 3.26.1-2ubuntu1