This is how it has always been as long as I have stayed at Marriott (for at least 5 years). Thursday and Sunday are bridge nights in that they count as weekdays if you do not stay through Friday or Saturday, respectively. If you stay on Friday, then Thursday counts as a weekend. This is the same for Sunday and Saturday.

Sometimes, properties will show higher rates for one of the bridge nights, but a lower rate than the weekday rate, if they expect higher occupancy (i.e. a large group has taken a lot of the rooms for a convention or a meeting).

I would make two separate reservations if you reserve Tuesday, Wednesday, Thursday and Friday together and Thursday and Friday do not show up as the weekend rate.