I’ve just set up a Plex server on Linux. I have been using MacOS. I noticed that when viewing the library that’s on the Linux server, downloaded YouTube videos do not show thumbnails or metadata when I’m looking at the library (in the GUI). The same file when moved to the Mac server, does show a thumbnail.
So there’s nothing wrong with the file – the thumbnail and metadat information is embedded in it.
All the settings for both servers are the same (unless I’m missing something).
I’m viewing both servers’ libraries with the same browser, same Plex ID.