I experienced this too. Reservations disappear, but the hotel itself has it.

Here are some things I learned from my years at Marriott.

1) Do not make changes to the reservation. Sometimes the hotel doesn't get the new details. Instead, if you can, cancel and rebook a completely new reservation.

2) When cancelling, make sure you get the cancelation email. Also, call the hotel to make sure it is cancelled.

3) Never use the Marriott.com website with multiple tabs. You can cancel or change one reservation, and wind up changing another. (Yes, I got that on video.)

4) Reservations disappear. Save your confirmation emails. Also, the confirmation email will have the original cancelation policy.