T-Mobile NSA/ENDC uses b2/66+n41 or n2/66+n71. When I force the device to connect to LTE and watch the CA, it will only connect to n71 (2/66/n71), even though other devices will connect to n41 no problem.
On T-Mobile's SA network, it will try to CA n71+n41 and use the n71 carrier for uplink. However, it is clear that the Zenfone 9 does not do this agg. When forced to 5GSA, the OnePlus 10 Pro will have a healthy 40-50mbps up, while the Zenfone will sit at about 2mbps up. This is because T-Mobile does not intend to use n41 for uplink at all, and n71 is required for proper SA operation.
Overall, I'm a bit disappointed, as this device in its current state does not fully support T-Mobile. I'm not sure if this behavior is fixable in software, or if the CA combos were baked into the modem like this. Either way, if it can't be resolved within my return period, I'm going to have to return it.