Problems with custom score order in NSW/Instruments dialog

Issue type

UX/Interaction bug (incorrect behaviour)

Description with steps to reproduce

Issue 1

https://github.com/user-attachments/assets/8bc3c195-be70-460a-adeb-7f0dba8a337a

Choosing 'Custom' in the 'Order' dropdown resets any ordering customizations you've already done.

1. Create a new score
2. Add 2 instruments, Piano and Voice
3. By default, Piano will appear above Voice in the score order. Reorder them so that Voice is above Piano.
4. Then, open the 'Order' dropdown and select 'Custom'
5. See that the order resets and Piano is above Voice again.

When selecting 'Custom' in this menu, the current order of instruments should be maintained.

Issue 2

If I manually select 'Custom' as the score order in the Instruments dialog of an existing score, the next time I open the Instruments dialog, it doesn't say 'Order: Custom' but instead 'Order: --', and the dropdown option looks blank.

Then, if I manually reorder instruments in the Layout panel, the order says 'Order: (Customized)', which seems redundant from the existing 'Custom' option.

In either case, the order should just say 'Order: Custom'

https://github.com/user-attachments/assets/862f80bc-4385-47b7-b798-91b265049580

Supporting files, videos and screenshots

See above

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

Nightly

Regression

I was unable to check

Operating system

macOS 15

Additional context

No response

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

No source files, tests, or entry points are named. Start by reproducing both cases in the Instruments dialog and trace how custom instrument ordering is stored and displayed; done means selecting Custom preserves the current order and reopening the dialog consistently shows Custom.