In some countries (common in Asia and certain developing countries), hotel credit card transactions are processed using a separate stand alone credit terminal (as opposed to an integrated module in the Property Management System).

There are situations when a prepaid reservation is made online (or via a GDS or Marriott Central Reservations), the credit card details are transmitted to the property. Once received, the transaction is manually keyed using the credit terminal. If the credit card is a Visa/Mastercard, the terminal in most cases might prompt the user to select the local currency or the currency for the country (DCC) where the credit card is based. Most of the time, due to laziness (or direction from management), the person doing the keying by default would choose the DCC option. In some countries such as China, the credit card terminal doesn't even have the option to choose!

It might help if you contact the hotel (not Marriott Central Reservations or Bonvoy) immediately after you make the prepaid reservation and tell them to choose the local currency when processing the payment.