I’m also 99% sure the MIDI PC thing is the issue, obviated by my line of inquiry and further validated by the fact that in @Bernard’s thread (that ended 10 days ago with the faulty assumption that @xtian82 had provided incorrect info) we addressed the very same issue.
MG3, Gig Performer and Morningstar MC6 misbehavior
In the above thread, we already solved the problem of MG3 chaning presets by setting “No MIDI Controller” in the MIDI patchbay. I’m pretty sure @Bernard and I also discussed the very same issue on the Gig Performer forum and the mods there posted an image referencing the solution from here.