SUMMARY
I have multi display configuration. Both are connected via HDMI.
Main is monitor and second is projector (usually turned off, but Plasma remembers it). I'm working on X11 session.

I logged in into my session and after some time I switched to another user (this user has mostly default configuration, for example desktop is not restored and has one desktop - no any virtual ones - got only different theme. Therefore I got empty desktop with no one application running (placed on task bar). I run System Setting (pined on task bar) and saw only marker this application placed on task bar, when I clicked into it I saw like it is raising on second not currently visible display. I clicked second time and application has been minimized.
The same activity I did on application placed on System Tray. It was panac-manager (gtk+ app.) and like for previous application also here its window appeared on not currently working display so invisible display.

Please notice that there is not easy way to get this application on main display, only I can use option "Move" and blindly catch window of this application on second display. Would be useful to have option: "move to display X" after clicking into marker placed in task bar. I reported this several months ago.

STEPS TO REPRODUCE
1. Log in into your session
2. Switch into another user
3. Run System Settings (pined on task bar)

OBSERVED RESULT
System Settings appears on not currently working so invisible display

EXPECTED RESULT
System Settings should appear on main display, currently working and visible, this one what just working on

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: YES
(available in About System)
KDE Plasma Version: 5.17.5
KDE Frameworks Version: 5.66
Qt Version: 5.14

ADDITIONAL INFORMATION