SUMMARY
When using multiple monitors, after unlocking after the energy saving timeout all the open windows are moved to the primary monitor even if they were in the secondary before the lock.

STEPS TO REPRODUCE
1. wait for the monitors to go off
2. press a key to show the unlock screen and type password

OBSERVED RESULT
When the session is back all the windows re-arranged in the primary monitor.
Also, applications in full screen in both monitors cannot see the second monitor again.

EXPECTED RESULT
When the session is back all the windows are where they were before locking (including the secondary monitor).

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Debian bullseye 5.10.0-5-amd64
(available in About System)
KDE Plasma Version: 5.20.5
KDE Frameworks Version: 5.78.0
Qt Version: 5.12.2

ADDITIONAL INFORMATION
This does not happen if I use Win+L to lock and then unlock before the monitor power saving timeout kicks in.