I recently book 3 nights in Cayman through American Express, to my surprise nights do not count towards your Elite Nights. I have not seen this rule anywhere.
Have you made the stay yet? If not, be sure to add your Marriott Rewards number when you get to the property. I am not aware of a reservation made through American Express not being counted as an elite night.
Was this a special package deal of some type?
If this is just a straight up reservation, the nights certainly should count towards your Elite night count. It does not matter what card you use to pay the bill.
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