Seing this in iOS app, both release and latest beta (build 13204).
The video file is an h.265 HEVC (mp4) file I converted from an h.264 file with Handbrake. The subs have been compiled into the video file (No external .srt file).
The subs are late around 8-10 seconds or so.
Of note, the Nvidia shield has no problem with the same video file with included subtitles. I also tried Windows Chrome (Plex web app), and Media player Classic, both play the subs in sync with the sound.
The same file plays in sync with the subtitles in Safari for iOS on an iPad (Plex web app), so I am pretty sure it the iOS app.
Art

I found some other issues but now everything should be fixed.