Here’s a couple threads regarding similar issues. The fix in both cases was to make sure both the Zwift app on the Apple TV and the companion app on your handheld are updated to the latest versions, and to make sure that you force-quit both the Zwift app on the Apple TV and the companion app on your handheld between activities. You can force-quit an app on the Apple TV by double-pressing the home button on the remote and then swiping up on the Zwift app window.