Out of curiosity, why would you spend points that aren’t yours? I have no experience with this issue but I’m guessing Marriott would cancel your new reservation to pay for your last reservation. It’s just a matter of time until the hotel’s accounting dept figures out that they weren’t paid.

If this happened to me, I would leave those points intact in my account. If they are still there after 3 months, feel free to spend them.

Please come back and let us know what happened. AFAIK, nothing like this has ever been posted on FT.