MIDI files - note events in Track 0
It is customary (although perhaps not required by the MIDI specification) that the first track in a Standard MIDI File (often called Track 0) contain meta-events (announce tempo, etc.) but not note events, controller events, and the like. Note events and controller events are customarily put in other tracks.
MuseScore, when exporting as a MIDI file a single-part score, puts the note events and controller events in Track 0.
As I said, this may not breach the prescriptions of the MIDI specification. But at least one of my MIDI editors complains about it.
Several MIDI-aware applications accept these files without incident.
Still, this format structure should probably be reviewed as to whether it is appropriate.