I have the 2016 PUS6501 Android TV. It plays subtitles correctly as long as they're included in the file. No external subtitles are played. Is this a serviio issue? Or the client player issue?
Later Edit: I tried some other clients, like Kodi and Movian. For Kodi subtitles work but ~3 minutes before the end of the clip, the subtitles stop working. I had the same issues with the webOS 1.4 native client when sometimes the subtitles stopped working after 35 mins on 45 min clips.
On Movian, external subtitles don't work at all, only internal subtitles. Same issues as with the native client. I still don't know if these are client or server issues. But I will gladly help to debug these.