This might not apply to you since you've only been charged one night on the prepaid*, but there have been instances where those who have prepaid don't get a folio & it's a PITA to get nights & points credited. So the unofficial rule of thumb is to charge something, even if it's a very small amount, to your room as it will trigger a folio.

* Sounds like they only charged you the first night as a prepaid. I'd expect you to get the 2nd night charged at check-out.

Cheers.