It may not actually be weird, if it’s related to this:
There’s a workaround discussed in that post which involves enabling additional metadata sources. To be honest though, I’m not certain if this is related to your issue. Others may have something to say there.
You may also want to clear your agent caches to ensure that they’re not interfering with proper artwork download:
https://support.plex.tv/articles/202967376-clearing-plugin-channel-agent-http-caches/
Beyond that, if you’re certain the artwork you want exists on The Movie Database, I’m not certain what else to try.
