According to Once Upon a Time in the West metadata - #4 by dane22 plex needed to remove some artwork due to legal requirements. I would (always) recommend using local asset artwork for your media → https://support.plex.tv/articles/200220677-local-media-assets-movies. That way you can fully control the artwork and you are not reliant on plex for it.
I’ve been trying to avoid that as it would make my file system much more verbose (is that the right word?).
Perhaps, I’ll have to do it for one offs and I’ll move to a separate folder (like I do for some with extras).
But for the most part, I have all my movies in a flat folder (mkv with embedded srt).
It’s a shame though, because using the old way (via tmdb) we get posters just fine. So this is a knock against using plex’s agent and seems they can’t do anything about it? (legally)