I think you're making a few assumptions that may or may not be true. Not in any particular order.

There is literally nothing in Marriott T&Cs saying debit cards can't be used at any MAR property, whether advance purchase rate or not.

Unless you've changed your booking (dates, etc), the rate will not change. Whatever you booked it at will be what's charged, whether it be in advance or at the front desk. The only variance will be if you change dates. BTW - that's always why it's good to have a copy of your reservation, to show the original T&Cs.

While advance purchase rates should be charged at time of booking, not all are. Some are done day before or day of arrival.

Advance charges can get declined for a variety of reasons: lack of funds, location of property, location of debit card (I know of a US property that has a Polish client & his debit card charges keep getting declined/it's outside his normal spending pattern or outside his limit, even though he says he has enough, & it's more to do w/ the bank's procedure & not the hotel's). Marriott's (and most hotel chains) are is if it clears it's good/doesn't matter if it's a debit or credit card.

I do agree that if your debit card was declined the property should have reached out to you, but they might have taken the approach of well it was declined but we'll just get a different CC at check-in & charge it then - aka, nothing nefarious.

Cheers.