I would agree with your assumption that the tower owned by AT&T just no longer has and agreement with T-Mobile.
Tower sharing has always been a game of industry politics and where coverage can be traded among carriers.
They will need more information to verify where it is and too look into it further.
But I would suggest calling in as it will be much quicker to get a response of some kind.
Future network plans are not released to the public so I doubt you will get an answer you are looking for other then if they still have an agreement and something is wrong or it was discontinued.