I've often needed to pay a deposit for a FHR stay, but paying the full amount 3 months in advance seems to be too much. I have an upcoming Four Seasons Orlando stay of 3 nights, and they took 2 nights deposit at booking!
I would call Amex Travel and find out what happened here. All FHR bookings are cancellable until close to arrival - I think Fairmont is the worst deal at 14 days out, most are only 1 or 2 nights before - so you can get a full refund that way if Amex Travel can't otherwise help you out.