By Default, Axis Studio it set to the Auto Detect working mode. This means that it will select your working mode based on the sensors that are connected to the software. If you try to use the Upper Body mode this way, the posture calibration will disable the T pose.
To fix this problem, before you connect the sensors to the software, go to Main Menu --> Settings --> Working Mode.
Change the working mode on the frequency you are using to Full Body mode.
Then connect your sensors to the software and perform the posture calibration. In the Posture Calibration window, you can now select A and T pose only.