I've had similar problems when switching between Android & iOS devices over the years with both T-Mobile and AT&T. It was a bit perplexing the first couple times it happened. Then I got used to the drill.
The solution, at least in my situation, was to call the carrier. Apple wasn't at fault.
Both T-Mobile and AT&T would fail to complete all steps required for the transfer, and therefore still showed the old device as the active one. A simple call resolved the issue.
In a couple instances, AT&T & T-Mobile would resolve the issue. Then I'd see the same behavior again weeks later. Sure enough, they had reverted to the old device on my profile. Calling them again resolved the issue.
The solution, at least in my situation, was to call the carrier. Apple wasn't at fault.
Both T-Mobile and AT&T would fail to complete all steps required for the transfer, and therefore still showed the old device as the active one. A simple call resolved the issue.
In a couple instances, AT&T & T-Mobile would resolve the issue. Then I'd see the same behavior again weeks later. Sure enough, they had reverted to the old device on my profile. Calling them again resolved the issue.