SUMMARY
A behavior change that appears to be new (at least since 5.18), is that the screen locker cannot be unlocked with just the keyboard - a mouse move is needed to show the password input and without it the screen cannot be unlocked.

STEPS TO REPRODUCE
1. Lock the screen (either by waiting for a timeout or clicking the META+L).
2. Press ESC or try to enter the password

OBSERVED RESULT
Nothing happens

EXPECTED RESULT
The password input should show and take keyboard input.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
KDE Plasma Version: 5.20.80
KDE Frameworks Version: 5.79.0
Qt Version: 5.15.2


ADDITIONAL INFORMATION
Moving the mouse reveals the password input with the keyboard focus (no click necessary). Interestingly enough, with a multi-screen setup moving the mouse to another screen reveals the password input the new screen but the keyboard focus stays on the first password input that does not hide. This is maybe not a great experience - sometimes I will move my mouse to another screen because the first screen didn't turn on properly (happens more often than I care to admin) so having a visible but inactive password prompt is not great, nor is having multiple password prompts. I remember lightdm used to "move" the active password prompt to the screen with the mouse.