Can't change clef
Note the misplaced bass clef on the second staff. Fix the staff clef to make it bass clef, delete the spurious clef, write it out, close, reopen.
It then goes back to the way it was.
OS: macOS 10.15, Arch.: x86_64, MuseScore version (64-bit): 18.104.22.16837, revision: 148e43f
The first time It tried fixing it, it crashed both MuseScore and my mac.