That's odd, indeed. If you install the external magnetometer it over-rides the magentic sensors in the ADARHS entirely, even if the magnetometer fails (the idea here is that if you have the magnetometer, you likely have ADAHRS-based magnetometer sensors that aren't very calibrate anyway, and in an emergency you still have GPS ground track).
In network setup, are you seeing both ADAHRS? Can you, from SETUP MENU > PFD SETUP > ADAHRS SOURCE SELECTION, see both ADAHRS? If you use the compare screen, does anything look amiss there?