I created a lead with a flute and added two one note harmonizers.
I had originally done it as one original, then one a fourth below, and one an octave below.
When I imported the midi into MuseScore, I ended up with two extra tracks with some of the notes from the other tracks on them.
I figured that maybe the range checking might have assigned them to a different instrument because they were out of range.
To check, I changed them all to just be 1-note harmonizers with no adjustments and the strange extra tracks still showed up with part of the notes on two extra tracks.
The one labeled Flute, L2: Flute-3 is the original starting with an A and the top two are the two harmonizers but part of their notes showed up on the two additional tracks.
I additionally tried raising the octave to see if that kep them on the same track but it did not. I also opened the midi with Logic and the same extra tracks show up so it’s not just MuseScore doing it.