Adding or deleting key signature in a part adds it to/removes it from that instrument only

Issue type

Engraving bug (incorrect score rendering)

Description with steps to reproduce

1. Create a score with more than one instruments
2. Add a few bars of notes in one instrument
3. Open the part for that instrument
4. Add a key signature a few bars in
5. Return to the score and note that the key signature has only been applied to that instrument (like a 'local' key sig).

Note that deletion suffers from the same problem in reverse:

1. Create a score with more than one instrument
2. Add a key signature a few bars in (in the score)
3. Open one of the parts, find the key signature there and delete it
4. Return to the score and see that it's only been deleted in that instrument

Supporting files, videos and screenshots

Adding

Score:

Key signature added in Violin I part:

Returning to score, key sig appears only in that part:

Deleting

Score:

After deleting the key sig in the Vl 1 part:

What is the latest version of MuseScore Studio where this issue is present?

3.6.2; 4.0.2; 4.1.1; 4.2.1; 4.3.2; master
[the deletion problem only arises from 4.2.1 on, as this operation was simply prevented in parts in earlier versions]

Regression

No.

Operating system

Linux (but presumably any)

Checklist

• This report follows the guidelines for reporting bugs and issues
• I have verified that this issue has not been logged before, by searching the issue tracker for similar issues
• I have attached all requested files and information to this report
• I have attempted to identify the root problem as concisely as possible, and have used minimal reproducible examples where possible