Um, that doesn't sound quite right, if you booked online. From the Marriott Rewards website under "How to use points." [Boldface added]:
"Cash + Points is available to book today for stays of two or more nights by combining redemption and cash nights when you book online.
Here’s how it works today.
Check “Use Marriott Rewards points” in your hotel search box. Then select your room type/points for your stay. Then check the “Use Cash + Points” box.
Use our calendar to find the combination that works for you
Coming March 24, 2016: Mix cash and points within a single night."
I've used points only, cash only, and points + cash and have always received the 5th night free. Did you book the room online with Marriott.com?
Exactly -- but you can pay cash for additional nights (on stays of >5 nights) and still get the free redemption night, providing you used points for four of the nights (as in hassmh's example).
I think the key here is what foxglove bolded:
If you're booking 5 nights, and choose cash for 1 night, you're paying for your free night, which doesn't make sense
If you are trying to book 5 nights, redeem 3, pay for 1, and get 1 free, it doesn't work like that. You have to redeem 4 to get 1 free. If you want to book 6 nights, you can redeem 4, get 1 free, and pay for 1, that would work. The key is 4 redemption nights to get the 5th free.