When you open the cover manager and have more then one poster, if you click on the move cover to first option it actually wipes the cover you are moving and copies the cover that is first in its place.
E.g open cover manager on film that has 2 or more posters (posterA & posterB) Now if you move posterB to first you end up with posterA & posterA posterB has dissappeared! Hoping you can fix this in the next release