Reasonable ofc yet how did they manage to clear their doubts for MC? HOW can magic determine with absolute certainty MC has amnesia? Are they keeping MC close to keep an eye on them? THAT'S a more sound theory for someone as cautios as they are.
I cannot tell you how exactly they have cleared up those suspicions for themselves as that would be spoilers, but I will say as follows. Although the MC has been formally questioned about anything they could remember (which was basically nothing), they have never directly faced any accusations or suspicions from the Gray Regent about that.