SUMMARY
As in subject. I have 2 displays connected to my PC. First is monitor (primary screen) and second projector, which usually is working in standy mode. Plasma detected both.
I tested this issue with two live distributions. I mean KaOS 2020.02 (Plasma 5.18.1) and Manjaro 20 (Plasma 5.18.4). Also I tested this  with older KaOS with Plasma 5.17. The same result. 

After plasma session was started I run System Setting and/or KInfoCenter and/or Konsole. Window of started application appeared on not primary screen. To use/get it I needed blindly catch and move (holding Alt) window to my primary screen or just turn on projector.

BTW. Unfortunately icon of running application placed on bar where windows are minimized doesn't have option "Move to Screen", whereas this option is present in context menu of title bar of window. Therefore isn't comfortable move window to other screen.
Sometimes after moving window to primary screen issue disappear.
Sometimes after opening Display settings in System Setting issue disappear.
I'm not sure what exactly it depends on.

STEPS TO REPRODUCE
Having two screens connected to PC.
1. Start live distribution like KaOS or Manjaro
2. Using Kickoff start System Settings or KInfoCenter or Konsole
3. 

OBSERVED RESULT
Window appears on not primary screen.

EXPECTED RESULT
Window should appear on primary screen.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: YES
(available in About System)
KDE Plasma Version: 5.18.4
KDE Frameworks Version: 5.69.0
Qt Version: 5.14.2

ADDITIONAL INFORMATION
mesa 20.0.5-1
xf86-video-intel 1:2.99.917+906+g846b53da-1
xorg 1.20.8