SUMMARY

Windows started having wrong behaviour on multi-monitor setup.  I have three monitors: 4K in front of me (primary), 1080p drawing pad monitor to the right, classic 1600x1200 monitor above the 4K.  From both lower monitors, many popup Windows and ones newly set fullscreen don't stay where they are rather than going to the top monitor, which they shouldn't.  If I leave the PC unattended overnight or even until the screen blanks, all the programs on the right-hand half of my 4K monitor (stacked) have moved to the left half, and the program on the left half has moved to the right half, and sometimes a fullscreen one on the 1080p has moved to the 4K.

STEPS TO REPRODUCE
1. Upgrade to KDE 5.23.n
2. Do menu things to open popups, set programs/images fullscreen, and for the other bug let the screen blank for a time.
3. Pop-up windows and programs set fullscreen move to the top monitor. In the other bug, programs move to the other side of the screen or other monitor.

OBSERVED RESULT

Wrong window behaviour (described above already).

EXPECTED RESULT
Don't move popups, newly set fullscreen programs, or ones snapped into half the screen, or fullscreen on non-primary monitor, somewhere else: keep programs, popups, new & non-primary fullscreen programs in the position the user originally put them and process the popups and new fullscreens at that location.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Slackware64-current / 5.23.1
KDE Plasma Version: 5.23.1
KDE Frameworks Version: 5.87.0
Qt Version: 5.15.3