The annoyance of Plex posters changing

I have tried a lot of forums and FAQs, but Plex keeps changing the posters I pick for a film which leave me no option but to keep changing them back or adding my own manually which is time consuming especially when I am replacing one that's already in the inbuilt selection.
I have just decided to create myself a watchlist and now I find the it just completely ignores any posters assigned, even ones I added myself and just gives it whatever poster it wants.
This is such a simple thing for them to fix, I have seen multiple posts on the subject and how annoying it is, yet it never seems to have been addressed.
Its supposed to be your own media the way you want it, but the look and feel is spoilt when you try and create some for of uniformity you find pleasing only for it to go and mess it up, for literally no reason.
What is the point of allowing you to select your own movie poster and then just randomly change it or for the watchlist to ignore it altogether even if you added your own.

Please someone tell me I have this wrong and how you can stop this, I have OCD and its doing my head in.