In my first message on this post I identified that I was not actually trying to book March 31, that I had identified this problem for the dates I was booking, had already booked 7 nights knowing I would be able to cancel in the future (meaning that this was not in any way a problem for me) and confirmed that I could see the same behavior on March 31 2019 in case anyone wanted to examine it.
I also noted that the Marriott CSR had confirmed after speaking to support to investigate the 'bug' that the restriction had been put in place "at the request of Revenue Management in the property and WAS permitted according the terms of the program."
The issue for me is the fact that people trying to book may be given the mistaken impression that the night was sold out, when they have in some cases paid Marriott a large sum of money to buy additional points for the supposed benefit of being able to book a single night at 13 months out if it is available - which is now NOT being fulfilled.