I understand why you feel AMX should be applicable since all the payments were made on the AMX card. However we find it rare now to be able to get the AMX applied to a cruise.

Any kind of discount, special or other SBC then AMX is not combinable. Very rarely are we able to apply AMX. Most cruises have a benefit that cannot be combined with AMX

Many are cancelling AMX card in favor of another card. Popular now is the Chase Sapphire Reserve. Triple points for travel and restaurants and a Chase air booking engine that covers many airlines.