I was Platinum Premiere on December 23, 2018 when I checked in for a 10 day stay bridging to 2019. I had 115 nights in the books on December 23. But I only had $18500 in spend on December 23.
Based on my room rate, I should have hit $20,000 spend on or about December 28.
But on January 2 when I checked out, I rolled over to 1 night for 2019 and all the fees charged from December 23 - January 2 got credited to 2019 spend. So still Platinum Premiere leval and no Ambassador status.
I called Marriott Rewards and they split the 10 day stay into December 23 -28 and then December 29-Jan 2 (the agent said "that's not fair to you") and bingo, in 5 minutes, I was now at Ambassador level. I heard from my Ambassador in about 4 days.