Hmm.
I'm not an avionics expert as far as antennae characteristics / water in the unitc goes. I was an intructor on the HT-1000 as embodied in the ATR, however I haven't flown it for many years.
Haven't heard of anything like that before. Yes, I've heard of the odd map shift. If it was having difficulty in receiving the satellites then yes its ANP could degrade, but you'd have thought you'd have received an UNABLE RNP message.
Did you happen to note what was happenning to the ANP?
pb