Yes, the networks will favor their own customers over their indirect customers that come through 3rd party resellers like Consumer, Mint, etc... because their SLA is written to let them prioritize their own traffic.

The regulatory framework is similar to when the baby-bells like Bell Atlantic, NYNEX, SBC, etc... were RBOC and third party CLEC started offering service. They could prioritize their own traffic when there's congestion but they can't deliberately impair those 3rd parties.

T-Mobile, btw, still has its own cellular network with a stronger focus on 5G.