Musescore suddenly only plays only one instrument

My son and I wrote a piece for four instruments (recorder, violin, and 2 cellos). Everything worked fine, but now I am in the situation that whenever I press the 'play' button it only plays the violin part. It doesn't matter which measure or note I select in which instrument, it will only play the notes or rests of the violin in that part. This is when the full score is open.

When opening individual parts and pressing play there, the recorder and cellos will play their own part, without the others, but the violin part will play both the violin and recorder, without the cellos

I thought it was a bug, I mean: obviously, because this makes no sense, but closing and reopening the file didn't fix it, so maybe it is a setting I set into the file without knowing it?! Has anybody seen something like this before?