I have had multiple bookings, at least 3, maybe 4, where I booked with points or free nights, and was still billed a random amount after checkout. This was in different hotel and different states. The previous times I called up the hotel and was told that they weren't paid by Marriott for the booking, so they billed me(?) or something to that note, and then reversed the charge.
Earlier this month I stayed at a Sheraton and paid in full with points, and found a $150+ charge on my card after checking out. I've called the hotel twice and they can't even figure it out. Called Marriot (not the hotel directly) and asked them why this keeps happening, but they try to deflect to the hotel.
I'm a Platinum member (so what, lol). Thinking of just disputing this charge and forcing them to provide proof as to why I was charged, but I'm wondering if anyone has had a similar situation and might have some insight.
ETA: Apparently this happens a lot.
TPG has an article on it.