SUMMARY
When upgrading an old Plasma installation (lets say 5.12 or before) - where the Display Configuration UI had a drop-down selection for "Primary Display", to Plasma 5.20 - where the Display Configuration has a checkbox for "primary" on each device, no device has the "primary" checkbox checked.

This can cause some weird issues when hot-plugging/unplugging monitors after the upgrade: if the "old primary screen" is turned off and on again, the secondary screen becomes the primary screen and does not reset back (see https://bugs.kde.org/show_bug.cgi?id=343631#c6).

STEPS TO REPRODUCE
1. Install Plasma 5.12
2. Set up multiple monitors with one of them selected as the "Primary Display" in the Display Configuration UI (i.e. make sure that the drop-down does not say "No Primary Display")
3. Upgrade to Plasma 5.20

OBSERVED RESULT
In the new Display Configuration UI, no device is selected as "primary".

EXPECTED RESULT
The device that was selected as the "Primary Display" in 5.12 should have its "primary" checkbox checked in 5.20.

ADDITIONAL INFORMATION
I kind of expect this ticket to be closed as "RESOLVED: who cares" and I'm mostly opening this for documenting the problematic behavior and the workaround - which is to open the display configuration and fiddle with the check boxes.