iPhone freezing during workout

New to cycling & Zwift but really enjoying it. My set up is Wahoo Core, iPhone through to Apple TV (mk2) screen mirror to Tv.
No problems with general Zwift Rides. However I’ve tried to complete 2 workouts & frustratingly as I near the end and tried to send a thumbs up on my iPhone the screen freezes and ruins the work out. 53 mins I to a 55 min work out! You can only imagine the language. Any advice or ideas welcome.