Hi, you have a mixed Plasma desktop: some parts are from 5.21.5, others from 5.23. Crucially, kdecoration is from 5.23 whereas kwin is from 5.21.5. This causes serious trouble.
I wrote about a possible solution here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=996726#10 -- Med vänliga hälsningar Patrick Franz
