Hi
The data register is used to write data to that is to be sent and read data that is received. That is, unless you have a loop-back where the data your send is received, you won't see the value that was written (you will see the last data that was received).
You should however see the written data being sent out on the Tx pin.
Regards
Mark
(uTasker project developer for Kinetis and i.MX RT)