This happened to me. Several days after final payment was made, July 27, I got an email balance due notice for $175.97 on August 1. I just paid it that one time. That began a series of balance due emails, for the same amount, every other day for about 6 weeks. That also began a series of AMEX credits for the same amount, six total. I was using a specific Celebrity Vacation Planner. She checked and said it was an IT issue and started working on it. Finally she called me with the resolution on Sept.19. She said I would show a balance due one more time and an immediate credit back to AMEX. That seems to have worked. No balance due emails since and she sent me a current invoice showing no balance due. She said she had heard of this happening before but never with one of her clients.
You just need to get who ever is handling your booking to get on it. Hopefully some of what I posted here will be helpful to him/her.
She threw in some additional OBC for the hassle.