I seem to be having the same issue mentioned in these places:
The issue seems to have been prominent in April and is listed as fixed but it’s still present in 2021.2.0b16 for me. I’ve tried to repro it for a bug report but somehow it’s not happening in an empty project with the same List structure. It’s happening in a non custom editor class, on an M1 Mac. NonReorderable does help. Any ideas?