I wonder if it is intended that overlapping screens are allowed in a multimonitor setup. I myself had not an issue with it so far but I got a report that a user had strange visual effects until he realized that the two screens where configured to overlap a few pixels which caused the effect.
Yes, it's intended. There's a snapping algorithm in place which helps preventing that, but the UI should really allow it. Thanks for checking!
This issue has actually been resolved in 5.27. See https://bugs.kde.org/show_bug.cgi?id=464760#c1