Intermittent FaceID Failure

My Iphone 13 is suffering intermittend issues with FaceID. FaceID can stop workign for days, then restart. this works fine for days, then stops working again. i have had two support calls open, been into an apple shop, followed by a rull restore, which worked. two days later, face id stops working. so i have done a full reset, erasing all content and settings........on restart, setting up, FaceID does not work again.....its v frustrating. the store diagnostics did not find anything wrong, its crazy, anyone had a similar issue and know what to do?