I had a similar thing for the Bangkok Summer Getaway (the staying period ends in Dec 2020 but I could book for March next year). I called Marriott explained the situation (I had already reserved my room) they put in a report and had the general manager email me. From there he did agree to honour what I was able to book despite the rate details showing the booking period as Dec ? 2020. This was kind of him as I am using it for hotel hopping (if I can even enter Thailand) for the extra points from the promotional rate.