Parts inexplicably mute

Working with MuseScore 3.5.1.13199, Rev 43c5553/Windows 7

I find that in a MuseScore score, certain parts are inexplicably mute. When I first encountered this (a few days ago), which ones were mute seemed to depend both on the instrument involved and the sequence of instruments in the parts above. In some case, for two successive parts with the same instrument, one part would speak and the other not. The pattern was not readily obvious.

But today I am not able to reproduce the situation in which whether a part speaks depends on the sequence of instruments. Very odd.

In any case, the attached score is annotated to show some instruments speaking and some not (as observed here today).

Doug