Disappearing notes when adding beats to measure with Musescore 3 score opened in 4

I have some real complaints about 4, but I like the features added, so I'm trying to use it and be patient, and I understand that some things may not work and there's not much to do about it. However, if I may complain: I typeset the psalms of Catholic Vespers for our organist, who needs a fully-notated accompaniment in lieu of using an antiphonal or Liber Usualis.

This means the number of beats per measure is often some absurd number, e.g. 28/8, and the tones change depending on the antiphon sung before and after, in order to get from the psalm back to the first note in the most harmonious way, so until now, I have copied the score file, opened it, changed the measures accordingly to add the new ending, and then I fix the harmonization (usually just changing the rhythm; the pitches are usually the same).

However, with the new version's changes, this resulted in the first notes of the subsequent measure being deleted, and cmd-Z doesn't restore them. (I'm on a Mac, Ventura 13.0.1). :(