Dan, I really don't know anything about MFS. I also don't know what you're referring to by "the bug on the tape" in mach mode. If you tell the aircraft to hold .82 mach it should try the best it can to hold .82.
You might want to ask somebody who is an expert at MFS because I think the differences between that program and the real thing probably out weigh the similarities. If you want to fly your sim like people fly the real 321 don't fly it right at it's critical mach in the real world that would be called dangerous