You might want to call T-Mobile's International Support line at +1-505-998-3793. It's a free call from your T-Mobile phone either cellular or WiFi.
I have not heard of your specific issue before but there are sometimes oddities when roaming in China.
Just checking in here to see if you were still having issue getting your data to work in China? If so, please call the number that drnewcomb2 posted so that our care teams can take a look at the account as well as the specific roaming agreements we have with our partners in China.
Did your problem solved?