Running Ubuntu 20.04 in extended display mode with 2 monitors. Driver version 470 (tested) from the Ubuntu repository (No further info is listed about it). GPU is GeForce GT 710. The monitors are: LG Electronics L226W (CRT-0) (primary) & LG Electronics W1952 (DFP-0).
When the lock screen timeout is exceeded the system locks the screen, and cuts off video signal to both monitors, putting them into power save mode. When I move the mouse to turn the monitors on it will sometimes revert the display to mirrored mode. This happens on the lock screen, and if you login the mirrored displays continue.
If I start typing my password quickly on the lock screen, before the second monitor starts displaying, then the system preserves the Mirrored display mode.
This does not happen with the open source driver
If I want to restore the extended mode I can do one of the following:
Go to Display settings. It will show as still in Extended mode, even though it is acting in Mirrored mode. Therefore, I must set the display mode to Mirrored and revert the changes.
Let the system lock with both monitors in power save mode, then attempt to login quickly in order to recognize the extended mode. (See above.)