I've had this happen a few times. Its entirely an IT issue with Marriott.
The original T&Cs apply to when you made the booking, but the system will just blanket apply whatever is the most recent/current info.

So you can get what is essentially a 'false warning' about a penalty when going to cancel a reservation that was made under terms that allow no penalty. I've actually not ever had an issue with this, since I canceled despite the warning and was not charged anything by the hotel... as it is up to the hotel to manually impose the cancellation fee and they can see the reservation terms.

Also had this when a property instituted a 'resort fee' after I booked. I asked and property confirmed that the fee won't get applied to my stay, and it didn't.