I have a similar issue coming up. I have a 7 night stay starting next week and ending after Jan 1. Depending on stays between now and then, I may find myself one night short of Titanium. If that's the case, I'd like to do a 1 night stay at another hotel in the midst of my 7 night stay. Based on what I've read, should I expect:

-The 1 night stay will post, because it is the first check-out day.
-The 7 night stay will not post, because the system will reject it after check-out because it sees a one night overlap.
-I can reach out to Marriott to rectify, will they credit me for 6 nights of the 7 night stay for 2021?? Or something else?