Clefs appearing and not able to delete bar

Hi, it happens sometimes, that "out of the blue" - when I change a Clef in a instrument, MuseScore creates visible Clefs for all other instruments, sometimes I am able to copy the music, insert a new bar, then delete the "wrong bar", but I believe there could/should be an easier procedure...and also sometimes I'm not able to do so.
What is this issue for and how can I prevent this to happen?
Thank you,
Emanuele