We are testing HEVC video, which Apple requires fmp4 HLS. We have this all working on Apple TV 4K with the exception of closed captions / sub titles. We cannot get the sub titles to display - using ismc1 captions (from a ttml source) on Apple TV, though we can get it to display in Safari in some cases.
It seems as if this is either buggy or not sufficiently supported currently? Even Apple's advanced HEVC fmp4 sample stream does not display captions (from 607/708) nor the webvtt based sub titles
Apple Sample stream - from https://developer.apple.com/streaming/examples/
Has anyone had success with this?