My cadence data constantly jumps around 20-30 RPMs up and down only in workout mode and no other time. I use the Garmin cadence sensor that came with my Edge 510 computer. The Edge and FIT files show correct values so it’s not my sensor. Support says they need log files but I use an Apple TV and they said Apple TVs don’t have log files available.
Bike is about 12 feet from the ATV and I use the CABLE ANT+ to BLE adapter. My power comes from a Powertap hub and I use the standard Garmin HRM. Again, cadence data in Zwift is fine when I’m in an event or just free riding - it only happens in workout mode, and it happens constantly. If it was a connectivity issue, I’d expect to experience it all the time, or at least some outside workouts.