Apple TV 4K 5.1 in apps not working

I have recently purchased an Apple TV 4K. All is set up and working. The Apple TV is connected to my Sony KD-49XD8077 via HDMI. My TV is connected to a Sony STR-KS1100 via optical cable. My Firestick, Xbox (and Sky Ultra HD Box when I had it) all work and play 5.1 perfectly fine. My Apple TV however is struggling.

On my Apple TV, under Video and Audio, Audio Format is set to Dolby Digital 5.1. My amp is happy. If I open the application called Surround Speaker Check (from the App Store) and test with Speaker Check 5.1 all works well, the correct speaker plays at the correct time. When I try and use YouTube and search for the surround speaker test video, I can only hear the front speakers. When I play stuff on Netflix with 5.1 sound only appears to come from the front. Is there something obvious I am missing? I have been googling the issue for several hours. I even tried making use of a little HDMI box which would convert HDMI ARC to Optical and it's the same as optical direct from the TV. Probably missing something glaringly obvious so any suggestions would be appreciated