I have the same issue and I think it’s a common one that hasn’t been addressed by Zwift. The suggestion always made that the cadence estimate being transmitted isn’t accurate is not the answer we are seeking. The problem is, the Tacx Smart Trainers transmit a Cadence Bluetooth signal, it is detected by all other apps that I have tested it with, without drop-outs, but with Zwift App on iPad, I experience frustrating drop-outs to zero, and in ERG Training Mode that causes frustrating changes in power / resistance from the trainer. So it’s not so much about keeping track of cadence as it is just making the set-up work consistently the way it was meant to.
In my case I also use Zwift Companion on my iPhone 8. I have had the odd session where this doesn’t occur, but I can’t pin-point the combination that alleviates the problem. There are a lot of devices in our lives and area of training that are transmitting at 2.4 Ghz, it’s the new way of life.
I have put in a support request too, but it seems a common enough issue that Twift and or Tacx should be addressing it together.