This is not a very serious issue, but maybe it could be fixed…
1… Go to Score Settings (Scores / Settings…) and uncheck:
“Key Changes for the Entire Project”
2… Make sure that the Display Transpose button is OFF (make sure it doesn’t look like the one below).
3… Add a Key Signature to a +2 semitones stave (let’s say we add “B maj” to a trumpet stave).
4… Turn on the Display Transpose toggle button (the 5 sharps signature will switch to a 7 sharp signature).
5… Try to change the signature of the same stave to “C# maj” (theoretically, the key signature should switch to a 3 flat signature, but it doesn’t).
RESULT: the Key signature doesn’t change.