SUMMARY
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Version: 5.18.4.1-0ubuntu as far as I can tell (The most up to date version on Kubuntu 20.04LTS)

STEPS TO REPRODUCE
1. Lock the screen from the menu
2. Type password
3. Press Enter


OBSERVED RESULT
Nothing happens, screen only contains the lockscreen wallpaper, no matter what I type or key combo I press. Lock screen is totally inoperable.

EXPECTED RESULT
Lock screen UI shows, focused on the password text field with the entered password

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.8.18
KDE Frameworks Version: 5.68
Qt Version: 5.12.8

ADDITIONAL INFORMATION
The only workaround I found is to have a pointing device connected to the computer, and input some event from it. Then the lock screen functions normally (AKA: to unlock, wiggle the mouse and then type the password).
This worked fine in Kubuntu 18.04LTS (without the cumbersome workaround)