I am seeing this issue regularly but more recently since I updated to 15.0.20 and 15.0.21.
A few times a day the Instrument Type/Clef suddenly changes.
When it happens the change is permanent. When I close and re-open the score window or the project itself, the incorrect instrument type is still selected.
A clue is that the layout dialog box shows the correct Instrument Type, but when you open the dialog box the current selected one is different.
Note that it should be Baritone and shows that here but it is actually using Sketch->Treble Staff.
Click on Baritone above but the dialog box shows the true (incorrect) setting:
