That doesn't make any sense.
The app only functions in IPv4. If the issue had to do with Apple iOS no longer supporting IPv4, then the issue would be the same across all carriers. Even if the carrier did support dual stack. The app still functions exclusively through IPv4.
It is possible that there was a carrier specific update that pushed alongside the 10.2 update, that prevents T-mobile iPhones from supporting IPv4 or Dual stack, but that would likely cause issues across a large number of apps. Many app providers have not yet implemented IPv6. IPv6 is not even available through all ISPs at this point (Many, but not all).