Had this happen with my St Regis Bermuda reservation. Booked via Amex FHR, but this is the first time I’ve encountered this issue and I’ve made FHR reservations at many Marriott properties. I kept adding it, sometimes it worked and sometimes it didn’t. I even managed to chat with the property ahead of my stay and then the reservation disappeared! It was quite inconveniencing because chat was the quickest way to contact the butlers; there was an email but there was quite a lag in response. Yes I could have called from the room phone but really missed the convenience of the chat!