went thru the exact same drama earlier this week with the westin rusutsu (hokkaido, japan) for a 2 night stay in late january.
i am a platinum with this sorry ... program.
you click thru and at the results page it shows points availability, but there isn't any when you click further. but the standard ("traditional") room no view are available for cash.
so i call, agent is just going off what their terminal tells them, i ask to speak to a supervisor, agent very rudely (ie "you're WELCOME!" when i didn't thank her for doing her job) put me on hold for 30, supervisor says, paraphrased: "marriott allows properties to black out high demand periods to a limited extent. this hotel in particular has high demand between 30 december 2018 and 22 march 2019. they can also limit the number of standard rooms available for points on a limited basis. i see here they sell points rooms until they have 5 rooms left. now they have only 3 standard rooms left"
being that marriott doesn't even have a platinum line (you get kicked into the same surly pool, at least in maryland) i don't even know if it's worth writing in to guest relations now as those standard rooms are now sold out. this is contradictory of the t+cs, and the stanza in section 1 about "we can revoke, not enforce, ignore, etc this contract at our whim", while written down and consented to, is likely not legally enforceable; you can't just write away somebody's rights, even if they agree. but i doubt marriott cares about this.
i'm going to drain my points and just go with airbnb or hilton, which at least seems to respect its diamonds consistently.