That is very interesting. My major issue I have been begging Plex to fix for a couple of years is that unknown subtitles will not auto start on Apple TV. The only way to correct is to use the old player. I wonder if these issues have a common root. Now we have the auto playing of sound issue. Auto playing of subtitles. And some people are having a problem auto playing the video at all.
It seems to me all those issue are very similar and the current hack solution is to use the old player. Which I cannot do because it would end up transcoding more content then I would like. I am a direct play or nothing person.