I am having low cadence readings too. I switched from a desktop Mac to Apple TV4K and picked up a 4iiii heart rate monitor to bridge my Garmin speed/cadence sensor to Bluetooth. My trainer is a Kickr Snap. Zwift looks much better on the ATV but I am not getting reasonable cadence readings. I can be pedaling easily at 90+ rpm but the reading on Zwift will be only about 65. Sometimes I can stop pedaling and the reading will remain at 65. Occasionally I can drop cadence to 40 and the reading will climb up into the 70s. If I shut Zwift down and start the 4iiii app on my phone, the cadence reading will be correct.
I cannot figure out where in the process, sending or receving, the problem exists so I don’t know who to contact.