In the case of the OnePlus 6T, only the T-Mobile version is ‘supported,’ when the unlocked version is the same in all other markets (including the US).

I’m seeing two models of the OnePlus 6T:

• 6T (A6013) This one is on the list of AT&T approved devices and most importantly has LTE bands 30 and 71 which are used in North America. source
• 6T (A6010) This one is made for the Chinese market and has the following LTE Bands: 1, 2, 3, 4, 5, 7, 8, 12, 13, 17, 18, 19, 20, 25, 26, 28, 29, 32, 38, 39, 40, 41, 66. Notice that North American LTE bans 30 and 71 are missing. source

Are you aware of a different 6T model besides these two or are you saying there are 6T (A6013) that AT&T are rejecting from activating on their network?