Altering the instrument in the mixer while playing causes that instrument to start again from the beginning while the rest of the piece keeps playing as normal.
S4 - Minor
Add two or more instruments to a score and add some notes.
Open the mixer, change the instrument from MS Basic to Muse Sounds or vice versa.
The instrument you change should start playing again from the start while the remainder of the instruments continue as they were.
This is a duplicate of #338189: Changing instrument in the mixer resets playback but not the playback marker.