Latest version 126.96.36.19990 changes all instruments to playback "Piano"!
I've just upgraded to version: OS: macOS High Sierra (10.13), Arch.: x86_64, MuseScore version (64-bit): 188.8.131.5290, revision: 5a0be2a
When reopening the piece I'm currently working on all instruments have changed to playback Piano.
I can then change all instruments back to whatever they were before (choir ahh, violins, violas, etc.), however next time I open MuseScore all instruments have again be changed to playback Piano.