From reading various threads here as to Marriott's promos with third parties (like Delta promos related to purchases of Marriott gift cards), Marriott committment to fulfillment and/or customer service has been, frequently, atrocious. While I have not had the experience, I have read posts in threads here complaining about various promotions in which Marriott and the third party have blamed each other, where the promo was never fulfilled, and where Marriott customer service has repeatedly failed to take ownership of problems such as you describe.
I do note in the rules that appear at the link the OP posted, that the pre-paid card company says it is not responsible for "late settlement" by Marriott properites. Of course, I don't know if such caused a problem here.