Spurious clefs, cannot delete (MuseScore 3 beta)

Attached are a small score and two screen prints.

The first print shows the score in normal view. Nothing special here.

The second shows the score with multi measure rests enabled. Now there are extra clefs in four of the five staves, that I cannot delete.

I can go back and forth, always with the same result.

Background: The score was originally imported from MuseScore 2. It had a few problems that I tried to understand by experimenting with the score. Among these issues were spurious clefs appearing at unexpected places. Most of these I could delete, but not the ones in measure one with multi measure rests enabled. I then constructed the demo score for upload by deleting everything but the first few measures. This demo still shows the problem.

Added: after closing and reopening MuseScore, the problem was gone.

I nevertheless report it, for it might be related to other problems with clefs I have seen reported on the forum. See for example, #279022. But there the suggested workaround was to delete the extra clefs, something that was impossible in this case.

OS: macOS 10.14, Arch.: x86_64, MuseScore version (64-bit): 3.0.0.19499, revision: 0526317