Yes, the indicated Mach showed values I would normally expect. Also, the TAS was available from ND, and that also makes no sense to me. Indeed, the sim used is rather aged.
One other thing comes to mind, real A/C will have a tendency to pitch continiously in cruise with A/P off, is this behaviour correctly replicated in the sim you use? Ours does not provide the high alt pitch-up and the onset of the (no airspeed) problem is then easily deal with by basically not touching anything. In real life, this is not an option I would say. Keep in mind that in ALTN law you cannot trim the pitch tendency away.
FD (the un-real)