I’m posting this in case anyone else gets this problem.
It applies when using Apple TV 4K with Polar H9 and H10.
I only solved it after getting Polar to replace my H10 which didn’t solve it and purchasing a new H9 which also didn’t solve it.
The dropouts were happening every few seconds at the beginning of rides. At first I though it was the battery which was changed multiple times and solved it generally for 2 or 3 rides. Then I realised that just removing the battery and putting it back in solved it, but only for a couple of rides.
Eventually, I worked out the bug.
These dropouts only occur when the HRM is paired on the initial set up screen. When the dropouts happen, if you stay on the ride and enter the menu, unpair and then re-pair the same HRM it stays good for the remainder of the ride.