I'm a plex pass subscriber running Server Version 126.96.36.19949 on FreeBSD (FreeNAS). The server has plenty of power to transcode multiple streams at a time. My tuner is an HDHR Prime with Cable Card and I'm only trying copy freely channels. I am able to play live TV from the browser, my Android phone and my Fire TV 2nd Gen. I'm not able to get Live TV working on My Fire TV 1st Gen or Fire Stick.
Sometimes I get an error that it failed and to restart the server and app. Other times it just goes back to the "watch now" screen or restarts the Plex app. When looking at the server status I see a line "Recording Live TV - Session" but I only see the "user started playing Live Session on Fire TV" When it works on my 2nd gen Fire TV.