Comment 5 for bug 1835863

I'd point my finger a the amdgpu driver in that case. Unplugging the monitor in question did the trick (allowed system to blank properly). The cable is definitely good & the connection ports on both ends should be fine. This particular system is dual booted into Windows 10 where this is a non-issue.

The keyboard has always been wonky (frequently it doesn't get initialized and has to be unplugged and plugged in again). I left it plugged in to run the test & the system was not adversely affected.

The same amdgpu is plugged into two monitors:

[1] monitor with issue (straight HDMI <> HDMI) FreeSync compatible
[2] monitor that works fine (DP on gpu converted to HDMI) non-Free-Sync