Changing Instrument behaves irregularly

When adding an instrument (in order to copy the music to the new staff) and deleting the former, the name does not change and can't be edited. There is some irregular behaviour in the "change instrument" stuff that I can't nail down to the core problem.
Example: Violin + Piano (see attached file "opus"). Add Viola, move it above the piano (I suspect here's the bug). Staff name shows "piano" instead of "viola" (in staff properties the entry is correct!). Copy first line from Violin to Viola staff. Remove Violin staff.
Goodbye MuseScore (crash).

Wish: Simply changing instrument in staff properties.