Comment by fiestajetsam

Apple TV and AirPlay doesn't support "out-of-band" of subtitles into playback or the use of a separate subtitle renderer, the subtitles have to be linked in the HLS manifest. For a lot of OTT platforms, subtitles are processed and handled separately from AV media, so this is difficult to fix. Some platforms work around this by doing manifest manipulation either server or device side, both have pitfalls.