It seems to get the Apple Workout App to register the Active Calories you must use the Apple Watch as the HR source. I paired the BT chest strap heart rate monitor to the watch and then selected the watch as the source before starting a ride. If you use the heart rate monitor as the primary source from within the Zwift App on the computer the Active Calories are not recorded by the Apple Workout App though all the other data is (Exercise minutes & duration). I prefer the BT monitor as it’s more reliable than the watch’s built in HR monitor which drops out frequently during a ride.
What seems really strange as the screen shot in my first post shows is that the Workout App can read the Active Calories data from the Zwift Companion App but cannot write this data to the Health App and therefore contribute to your ring data. This is despite all data sources from Zwift Companion being selected for the Health App. As previously mentioned everything works fine, including Active Calories when using a BT Heart Rate monitor, if using the Zwift app on the iPhone instead of the computer as the primary device.