Can't delete key signatures if at mmrest

Might be by design, workaround is to delete them in the main score

Regression vs. what version?

Hmm, 2.3.2 does allow that, 3.5.2 and 3.3.4 do not. So it is not a recent regression for sure

Another workaround is to apply a C-Major key (no flats or sharps) sig to the part

Which reveals a problem: that applies to that part only, not to other parts of the score, so makes it a local key signature. That part is not a regression though, same in 2.3.2, and trying to prevent this actually might be the reason why it doesn't work anymore in MuseScore 3

Can you explain in more detail - sample score, steps to reproduce? I had no problems removing a key signature in a part in a score I just created - worked fine both at the beginning and mid-score.

I could reproduce with a score of mine, Closed Score SATP + Piano, with a SATB and a Piano part, and with one flat.
And also from scratch, using the attached

Go into one if the parts, select the flat, press Del -> nothing happens

As said earlier: this might well be by design. I see good reasons for disabling it, at least.

I can reproduce with this, not my test - the difference seems to be mmrests. And I can delete the key signature in your test if I first disable the mmrest. So, perhaps a duplicate of this #46781: Unable to delete key signature when multi-measure rests are enabled, which is a very old issue? Unless it was at some point fixed then returned, which is certainly possible.

Strange though that it works in 2.3.2?

Actually it is the same in 2.3.2

So not a regression and another workaround: toggle mm rests

That's not really needed, but would solve this too