The Plex app on LG TVs cannot pass TrueHD or dts-HD audio via HDMI-eARC to attached audio equipment. This is a TV, not Plex, limitation and applies to all such apps, not just Plex. See this post for additional details.
TrueHD audio will be transcoded to a compatible format then passed via HDMI-eARC to the audio equipment. Any Atmos information is lost during the process.
For dts-HD MA audio, the core dts 5.1 audio will be passed. The “-HD MA” portion is discarded.
Note that the limitation seemingly applies to all TVs, not just LG. I have not seen any post on the forum where a Plex app on a TV can pass TrueHD/dts-HD via HDMI-eARC.
If you wish to have TrueHD + Atmos audio, you have one choice, a Nvidia Shield Pro. It is the only known off the shelf streaming device that can passthrough TrueHD + Atmos audio and play 4K HDR video. See [INFO] Plex, 4K, transcoding, and you for details.
