I have the same issue. I can see subtitles from a web player, but on FireTV they are not appearing (nor can I find an option in the GUI to enable them). They work fine for Movie playback (MKVs with VOBSUB), just Live TV (HDHomerun) has this issue.
Yes, I see new features but subtitles still were not working for me. So then I check on my Plex server and found a newer update. Installed that and now I’m getting close caption support! Thanks!!