Your season folder names are not what plex would expect looking at your screenshot.
Remove the year and season names from the folder names and try the plex dance again afterwards.
In addition, have you EVER manually selected artwork? If so they could be now be considered locked fields by plex. If so check this doc Reset all manual poster selections in a library at once or unlock other metadata fields
I personally use show, seasonxx and fanart (all jpg format) and they work perfectly.
