Out of my ass I would say that this has to do with kde trying to optimize the full screen window by skipping compositing for it (direct scan-out, useful for e.g. gaming), of course this is not possible to do with windows on top (they have to be composited). I would have expected KDE to disable direct scan-out once you put windows on top, but maybe it doesn't.
I recommend posting this to KDE's bugzilla, they should get in touch pretty quickly for something like this, of course recognizing and solving the issue are two entirely different things.
But before posting, I'd suggest you do a test: you can disable direct scan-out globally by going on "display settings" and unchecking "screen tearing: allow in fullscreen windows". Try and see if the issue goes away. And make sure to mention this test and the result on the bugzilla post.