Comment 4 for bug 1881387

I'm able to reproduce this issue on my Desktop computer (running Xubuntu 20.04.1).

My setup has two monitors: one connected by DisplayPort, and another by HDMI. I can only reproduce this if they are not in mirror mode.
One thing to note is that whenever the DisplayPort monitor is turned off or on (or disconnected), the screens are reconfigured.

To reproduce this issue, I lock the screen (through the screensaver or by invoking xflock4), and then turn the DisplayPort monitor off. After the reconfiguration the desktop is visible, although it's not possible to interact with it.

At this point turning the DisplayPort monitor back on triggers another reconfiguration and the lock screen is visible again.
When the desktop is visible through the lock screen it's possible to type the password and unlock the screen.