You may never know why your reservation was cancelled by Airbnb. My guess is that it has nothing to do with tiers, but rather the guest is in violation somehow, like setting up a duplicate account, having a history of wanting to talk off-platform, etc. It also might have something to do with booking over the holidays because of fear of parties, although it seems severe to deny such a long booking since parties tend to happen with the shorter bookings.
Anyway, it’s annoying and frustrating that the culture at Airbnb seems to be to provide as little information as possible in the face of severe, unilateral decisions. I presume that has something to do with our lawsuit-happy world.