In the past Marriott has always given the night's credit for the year in which the night was stayed, not billed.
Example; Check in on Dec 29, 2017, Check out on Jan 3, 2018.
- The following nights are stayed in and credited for 2017: Dec 29, 30 and 31.
- The following nights are stayed in and credited for 2018: Jan 1 and 2.
If you are concerned, out of an abundance of caution check out on the 1st and immediately check back in. I have done this many times when separate folios are needed to differentiate for billing or so personal and business are not on the same bill. In dozens of times doing this the hotel has never asked that the room be changed.
To be honest, despite a rule against it the hotel has often given a second round of Welcome Points. At the VA Beach SHS when doing this I pointed out I understood there was no second round of welcome points and the manager, who was at the desk at the time, said, "What the heck..." and awarded them anyway.