This happened on AC Atocha, Madrid, a couple years ago - all rates incl flexible rares, had nonrefundable in the rate details.
I called the central reservation line and asked the agent to check - she saw the same thing and claimed must be special event related. Then I asked her to check AC Carlton, another property no more than 10 min walk incl waiting for 2 traffic lights, what about the rates there. - I already knew at AC Carlton the rates were showing correctly i.e. flexible rates were flexible rates. She then said she would submit a ticket and told me to check a few days later. 72 hours later when I checked again - the AC Atocha rates had returned to normal.
This cannot be resolved online, have to call either the Marriott CS, or the property directly. The latter might actually be easier given the current state of Marriott CS is.