If you are manually choosing your own poster/fanart, that act should lock the poster in place. I’ve only seen locked items get changed under 2 circumstances.
1 - The movie/show is unmatched. Instead of fixing a bad match, the movie/show is left unmatched and the metadata is manually edited. In this case, Plex will try tomatch the show the next time it updates the library. If it finds a clsoe enough match, it generates a new entry in it’s database to add all the data, then deletes the current entry, thereby removing any customizations. The fix is to fix the match or if there is no match, in the advanced search options, set the agent to personal media for that movie/show.
2 - I’ve seen a bug in some 3rd party scanners that can cause this. If you are using a 3rd party scanner, I would contact the dev of that scanner to see if they are aware of the issue.
