There was a recent change in the default movie poster art provider to Gracenote instead of primarily TMDB and that’s probably what you saw impacting large number of your posters recently.
You can check some of the linked threads there for some details and solutions to help clean it up - someone posted a script you can run to set all posters to the TMDB default for example.
It was kinda annoying to go back through and fix artwork after that change but if you weren’t setting artwork manually already it always had a potential to change from TMDB user votes on poster artwork; which wasn’t often really. This change with art providers was more dramatic because of it’s sweeping changes (some good, some bad).
What tom80H says is really the only way to “lock” artwork though. Either use local artwork or manually select something and it will stick otherwise it uses the “default” and whatever poster occupies that default slot. At least that’s how I think about it.
There’s ways to automate the local artwork stuff using scripts or apps; Dokuro has a pretty robust system if you look around for their posts. FileBot is what many people like to use.
I have about 1000 titles and I mix and match “locking” artwork and not worrying about it - I only get particular about marketing copy or actor names which Gracenote has a LOT of and TMDB\TVDB never allowed so that was most of my “fixing”. I often use PlexDash on my phone to do quick fixes when I notice artwork is off when browsing from TV interface and it’s particularly useful for going through background art in a library conveniently (you can set artwork browsing to the background image and quickly change it that way) - which is what I did recently after that Gracenote change.
Anyways - thought that might help with the artwork stuff. The changes can be annoying, but in the past it was an advantage when artwork was less quality controlled to get the “good” poster when it updated but now it’s a little different and Plex is handling it very different so maybe the local artwork stuff is the best option for more control if relying on Plex for quality is less reliable.
