Newly added season posters aren't automatically selected, and get removed after the next refresh

The old tv show metadata agents will very likely see no further bug fixes. Because there is a new tv show agent being developed right now. [BETA] New Plex TV Series Scanner
If you are using English as your metadata language, you could switch to this agent already. If not, I can’t recommend doing so, yet.

In general, I strongly advise you to place any custom posters as files into the folder structure of your media. Do not use the drag&drop/upload methods to upload posters in the plex web app.
Please see the documenation on where to place the files and how to name them: