I have two setups that are very similar and exhibit the same problem:
Apple TV 4K HDR -> TCL 55 6-Series -> Optical Cable -> Onkyo S3500 Home Theatre System
Apple TV 4K HDR -> TCL 49 5-Series -> Optical Cable -> Vizio SB3651 Home Theatre System
In both cases, the audio lags slightly behind the video when set to output surround sound. If I change anything - so it's just stereo or receiving PCM audio from the Apple TV it goes away. On the 55 inch TV I have a splitter that creates dedicated HDMI Audio channel and that works fine, but the loop thru from the TV there's a lag. This only happens inside of Channels - Netflix, etc are all fine using the TV's optical out.
There are workarounds, but any of the workarounds ends up losing the 5.1 Surround Sound. Thoughts?