Notes missing from staff 5 on musicxml import

Describe the bug
The notes in the fifth staff of an instrument are not imported correctly on musicxml import, and are instead replaced by invisible whole note rests.

To Reproduce
Steps to reproduce the behavior:

1. Create a new file with an instrument, and add staves to it until it have 5 staves.
2. Add a few notes to each staff.
3. Export the file to the .musicxml format. If you open the .musicxml file with a text editor, you can see that the information about the notes in the fifth staff is present.
4. Open the .musicxml file with Musescore.

Every measure of the fifth staff in the opened .musicxml file is filled with an invisible whole note rest.

Expected behavior
In the opened file the notes of the fifth staff should be the same as those of the original Musescore file.

Screenshots
Original Musescore file:

Imported .musicxml file:

Platform information

• OS: Windows 10
• Musescore version: 4.0.1-230121751

Additional context
The same issue happens if the file is imported with Musescore version 3.6.2.548021803.

Start by reproducing the MusicXML import with a five-staff instrument, comparing the exported file and the imported score. Trace the MusicXML import entry point for the fifth staff; the work is done when its notes are preserved instead of being replaced by invisible whole-note rests.