Odd. I was able to reproduce this in some cases where the original poster art was never locked and was the default poster art. Also if you attempt to do it again, it won’t work a second time.
This issue was the result of the Beta Plex Media Server 1.40.1.8111 where Analyzing a file would change the poster and background art to an image from the video. This build was quickly reverted then a new Beta release was pushed out Plex Media Server 1.40.1.8120. Linking to the forum thread where this was initially reported for possible solutions to get your posters back. Some users used a backup and another has script automating the selecting of a new poster.
