One way to be sure it doesn’t do this is to manually choose you artwork for each item. This will lock in your choice, especially if you upload custom artwork from, say, theposterdb.com. Yes, this can be a real pain.
Plex updates poster art to stay “fresh” and assumes you’re ok with this if you never set a custom poster yourself. So, as I said, manually choose a poster for the media, and it should remember this choice moving forward. If the default choice is what you like, select something else, then switch back to make sure Plex knows you intentionally selected it.
go get ‘em!