Rest autofill is incorrect for 12/8 on time-stretched staff.

Issue type

Engraving bug (incorrect score rendering)

Description with steps to reproduce

1. Create a file with 4/4 time and two staves.
2. Change the second staff (only) to 12/8.
3. On the 12/8 staff, add a rest of any type. It autofills the rest of the bar like 3/2 instead of 12/8.
4. Undo that and add a note of any type. It autofills the rest of the bar like 12/8 (i.e., as designed).

Supporting files, videos and screenshots

https://github.com/user-attachments/assets/38f21870-b9cd-4a25-8897-9fde4a2e1441

In which versions of MuseScore Studio is this issue present?

4.6.3

Regression

I was unable to check

Operating system

macOS 14.7.8.

Additional context

Here is the test file I was using.
testfile.mscz.zip

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