the 4k or 1080 is not the issue, you are correct, however the link above will does explain potential problem and solution.
TRUEHD regardless of 4k or 1080 or 720, cannot be passed through or direct played with a tv or through a tv app.
Unless you have an nvidia shield connected to an a truehd/atmos receiver/soundbar, then truehd will always be converted.
this alone is not normally an issue, you can still direct play the video while the audio is being converted.
You do not mention it anywhere that I saw, but if you have sub titles enabled, then this causes another problem.
When converting the audio, and subtitles are enabled, then plex will transcode the VIDEO too, to keep everything all in sync.
You have 2 solutions.
- change to non-truehd audio and/or remux/remove the truehd so its not the first audio stream
- turn off subtitles
There could also be a problem with the LG app itself, I am not familiar with it, I do not have an LG tv, but there is an ongoing update posted about the new app @ New LG webOS App Preview which you can try or not.
Otherwise, there is not much else anyone can do to help unless you start providing logs reproducing the problem and maybe some screenshots of the plex server dashboard to see exactly what the server is doing to the stream.
