Mixer resets to piano

Installed Version 3.0.3.5734
Only one soundfont = MuseScore_general.sf3 installed
I "Save to score" before saving the score
When I select instruments in the score all works fine.
When I close MuseScore and open it again, all instruments play piano, although in the mixer, the correct instrument is shown.
I have to reselect the correct instrument again, then it works, but still is lost every time I open MuseScore new.
Any idea how to avoid this.
Thank you very much

Michael