I tried the new calibration function today for my Flux2 Smart. There wasn't any guidance or instructions. I first noticed a calibration button before starting a ride. However, when I selected calibrate, an explanation window popped up with a calibration button. After pressing it, nothing happened and the BT connection to the trainer was lost. I did a ride after reconnecting. At the end, I went to the device page and tried to calibrate again. Had the same result with BT connection lost. After that, I connected an ANT+ antenna, disconnected the BT transmitter, connected to the Flux2 Smart using ANT+. After doing that, I was able to successfully calibrate.
Not sure if calibration is supposed to work with BT or only ANT+, but currently it is not working for me with BT.