You might want to investigate more, looking into the files inside these folders:
$HOME/.local/share/kscreen/ # files with monitor layouts
$HOME/.local/share/kscreen/outputs # files with monitors
It seems that monitor IDs in layout files, stay the same, while monitor names may change (depending on various reasons, like GPU connectors, drivers etc., I guess).
You may want to try deleting one, or more/all files, to see if this fixes your issue (because they should be regenerated fresh).
I would archive the existing files, for later comparisons with the new ones, to possibly assist the devs find the source of the bug.