I've had the same problem. I escalated it to the point of Marriott's executive office. The lady I corresponded with insisted that even though the cancellation showed +0 points and my balance didn't change at all, I received my points back because that's how the system works so it must have.
After a month of back and forth I gave up, assumed Marriott was going to keep my points, cancelled my existing reservations, and have 0 nights to date this year. Tomorrow night's at a Hilton.