It's a shot in the dark, but submit a ticket to Niantic and see what comes of it. Emphasize that with MM being an exclusive move you can't just change it back.
Reasons like this are one of many why the exclusive move system completely bites ass, even if some of us have resigned to the fact that Niantic is likely unwilling to deviate from TM-locking them for any circumstance. Also the UI for selecting what charge move is getting replaced is confusing as hell and needs a 2nd confirmation prompt so you don't erase something unintentionally.