Created attachment 127405 [details]
Kernel log extract, showing the unblanking failure.

Setup: RX 470 with two monitors attached – one via DVI-and one via HDMI. (Both
are AOC monitors.)

* Ensure that outputs are blanked (via xset, time-out or similar).
* Switch off the monitors.
* Unblank the outputs.
* Switch on the monitors.

Invariably, in this situation, the DVI-connected monitor is unblanked and
displaying what it should be, but the HDMI-connected monitor is receiving no
signal (or at least nothing to display) and is therefore displaying nothing.

The HDMI-connected monitor can be restored by using xrandr to change mode or by
blanking and unblanking – so long as it's switched on while doing so.