It seems its CLIENT related indeed - the LG tv’s implementation of the Plex app simply barfs when hit with 4k HDR content with audio transcoded from DTS. I have tried multiple different computers as Plex media servers - all hardwired - so its not a CPU or bandwidth limitation which is what I initially suspected. Interestingly the audio works perfectly, but the picture is just colors and glitches.

Unchecking ‘DTS’ from the supported audio formats in the plex app causes the Plex Media Server to helpfully try to transcode the audio, which results in these exact graphical glitches as pictured at the top of this thread from about the ten second mark in any video. If you tell the LG WebOS Plex App that DTS is supported (configurable in settings) then the video plays perfectly - but without any audio at all (the content in question is DTS-only, so cant choose a different audio channel). I have an LG CX and have exactly the same thing playing 4k HDR content with DTS Audio via the LG WebOs Plex App.

Lg plex app 4k