SUMMARY
When unattended-upgrades is running a scheduled update for your system, which results in a shutdown lock, or a different background service has a shutdown lock in place, when the user tries to shut down from the Plasma Desktop the session reaches the point where it should end (the black screen) but the session never ends automatically, resulting in the user becoming stuck in that black void indefinitely until they go into TTY or force a power off by other means.

STEPS TO REPRODUCE
1. Enable unattended-upgrades, have some updates available and turn on the computer on a day that unattended-upgrades is scheduled to run on
2. Log in, and then try shutting down from the desktop

OBSERVED RESULT
The desktop session never ends, after getting to the point where it's ready to immediately end the desktop session, leaving users with just a black screen and their cursor left from what is left of that desktop session.

EXPECTED RESULT
The session will at least end, and/or the desktop will stay running until the shutdown lock is no longer in effect in which case it should THEN start the part of the log out process that quits plasmashell, etc.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: KDE neon 5.18.2
(available in About System)
KDE Plasma Version: 5.18.2
KDE Frameworks Version: 5.67.0
Qt Version: 5.14.1

ADDITIONAL INFORMATION
If I recall correctly, GNOME doesn't have this issue but it does require authentication to shutdown or restart when a shutdown lock is in effect.