350 nights obviously isn't enough on its own to get LT Plat. Were you one of the few that received LTP through being an original Marriott Rewards member? If so, I'm guessing the issue is their system may not be dealing with your account correctly as you don't have the requisite number of nights needed.

I would suggest reaching out to one of the Bonvoy Lurkers to see if they might be able to help you.