Any update on this? It seems to have been a problem a few months ago.
I am using a Wahoo Kickr Snap. I actually managed to use the Zwift Spindown Calibration once successfully - I think it was with the Snap connected directly to the Apple TV.
Now I use the companion app because I have three bluetooth devices - all are connected to the companion app and then the companion app connects to the Apple TV Zwift app.
The issue now is that the calibration never finishes - the on screen speed display stops at a variable low speed (last time it was 4km/hr) and the calibration never completes as a result. This is despite the wheel being completely stationary.
I am using the Wahoo app, but it is a bit of a pain to have to swap apps, deal with bluetooth disconnection and reconnection twice etc…