Bug in grace notes before backup handling
S3 - Major
The grace notes are attached to the C in staff 2 instead of to the G in staff one in this example.
So? Not in the attached.
I imported your xml (which stems from Sibelius via Dolet apparently), corrected the grace notes and exported to xml again.
It then imports correctly (read: as you want, gracenotes on top staff) back into MuseScore.
Are you sure in Sibelius they were attached to the top staff?