While I sympathize with the OP, it is time for me to make my annual explanation. As a part of schedule changes, a "repair" algorithm is run to fix itineraries which are no longer valid. This includes both flights and seats. Unfortunately, these algorithms do the best they can but are not perfect by any means.

The key is to check reservations frequently and be proactive in managing your reservations. While it would be nice to be notified every time that seats and/or flights change, it just doesn't happen. The best thing is to check reservations monthly until 90 days out and then check them weekly.