I have discovered that the Plex app in Android TV on 2 separate Sony TVs won’t play a flac file that has no cover art. The Plex app in roku and Android phone will play the file just fine. I confirmed with a couple of different flac files. The flac files are typical CD rips.
Do you mean cover art embedded in the files? I’m not having this problem on an nVidia Shield, which is also Android TV.
You can get the Plex Server version in Plex Web by going here:
You can get the version of the Plex app on the TVs by going into settings. The version will be at the bottom of the screen.