I just wanted to offer some help for those having subtitle playback issues on emby firecube tv app.
I have 2 firecube tv's with emby app
Playing video with subtitle can work for 30 seconds to a few minutes then stops entirely or becomes out of sync.
Subtitles work perfectly on s9 android app and computer browser playback
Since I know the subs are ok I figured it must be the playback.
I installed VLC player for fire tv
Went to Emby settings and enabled "External Video Playback"
Tested and confirmed subtitles worked perfect even with skipping back and forth in the video they stayed in sync.
Until there's an update it works for now.. I hope that helps someone.