I just checked the various Marriott account web sites. I was PP in 2017, and had similar patterns as last year so would expect to be PP in 2018 (but who knows).

As of just now:
iOS App thinks I'm still PP, and shows 2018 nights-to-date.
Regular web site thinks I'm still PP, and shows end-of-2017 nights earned.
Mobile web site (in chrome on iOS) thinks I'm regular Platinum, and shows 2018 nights-to-date.

Is anyone who's been "demoted" seeing it anywhere but on the mobile (non-app) site? Are ANY PP's seeing their correct status on the mobile site? Of not, I strongly suspect this is just a glitch with the mobile site not knowing about the existence / being able to display PP. In any event, since status is good to the end of February, any demotions are clearly premature.