Stop Plex changing posters?

I have the latest PMS installed on Windows. It runs really well but has a really annoying habit of automatically changing any posters I’ve manually revised back to the default. I have the automatic meta checks turned off but any time I click into a title that I haven’t looked at in some time it just refreshes it anyway and changes the poster. It just seems very unnecessary. Anyway, any advice would be appreciated.

What kind of content is in there? Regular Cinema movies or Home movies?

Do you have all your movies together in one folder? Or are you using one subfolder per movie?

Is this an older library which has posters from a now defunct metadata provider?

This was a new Plex install probably 2-3 months ago, started from scratch. Commercial movies only (ripped from Blu-Rays), all in their own folders. I don’t think I’ve changed any of the agent sources from their defaults.

Did you upload your own posters or did you just select a different one from the posters on offer, when you edit the movie?

They were all just selected from what was on offer. PMS decides periodically to reset them back to default (but only when I click into the film on PMS since I turned off all the auto meta udpates).

Does Plex usually match your movies at first try
or do you have to “convince it” with ‘Match/Fix Match’?

Are your media files stored on external drives or network shares?
Settings - Server - Library - ‘Empty trash automatically after every scan’
should be OFF under these circumstances