Hey Jofemodo!
The unit is in “stage” mode with the midi from channel one ( track one in the mixer ) cloned to other channels. If a note is held down on track one and you switch to another channel in the mixer ( which easily happens if you click on the screen bringing another channel into focus ) the midi buffer for channel one doesnt reveive the note off and then you have a hanging note.
If a track is disabled then all notes off should be sent.