There is no rhyme or reason to this. When I go to linked devices on the LG the Plex app just is there sometimes and others its not. And sometimes I will be right in the middle of watching something and it will just turn off and disappear. i don't know why this is happening. So I can usually go into the computer folder and continue watching the movie from there but sometimes that doesnt work properly either - the folder is there but the movie that I just recently placed into the folder is NOT. Even though it is in the folder on my actual computer - so I thought I would include that info just in case it gave someone a hint as to what is going on...
****** I just discovered that the plex media server is disappearing and reappearing in my media devices folder too in my network on my computer - so this is not an LG problem - it is something to do with the computer and the plex program. i reinstall or repair plex and it comes back into the media devices folder for a while then disappears again...