I recently had the same issue with a 001 ticket where an AA flight got a new flight number. It took a few days of back and forth on Twitter but they managed to sort it in the end and I’m now able to access BA MMB again.

Not sure exactly what they ended up doing but it wasn’t just a ticket reissue as they first tried that to no avail. When I phrased it like this they finally got it right.
“To me it appears as if the BA PNR hasn’t been updated with the fact that I’ve accepted the changed flight number on the AA side.”