If you have a look at my post here you can see that the issue is just on the LG and covers lossless 7.1 and some 5.1 audio. I can give your suggested steps a go this evening to log the errors on the movies I have in my post.
@GoodSpaceman Have you tried with a Dolby TrueHD 7.1 Lossless audio track? It appears it isnt just DTS that plex struggles with. If you look at the dashboard when playing you will see that if you are using the plex app on the lg tv, plex will transcode all lossless audio to AAC or EAC3. I have a hunch that the tv cant passthrough lossless audio. Unfortunatly I dont have an eARC compatible reciver to test passthrough over eARC 
