After hours of troubleshooting myself and also reading a lot of forum entries, it seems clear that when using the NVIDIA Shield as a live/DVR TV server, subtitles do not work.
They work on other platforms such as Windows but not Nvidia Shield with the Android TV OS.
It’s important to me as I have people in my household where English is a second language and Subtitles on Live and recorded tv can be critical.
I have them when using the HDHomerun app (my tuner is a HDHomerun quatro) but plex is a problem.
I am in the same boat. If I watch it from the recording the shield knows that the subtitles are there but will not display them. If I want to see the sub titles then I need to download the old TV show, encode it while also pulling out the sub titles into a .srt file. I have my account set to ALWAYS display captions/subtitles and it plays just fine with the .srt file. However, I don’t like having to download every recording of Bonanza, Love Boat, Threes company, Family Affair, Dick Van ■■■■, pull out the sub-titles, turn it into a .mkv, watch it only to then delete it.