I may have a temporary fix for now;
Add the title again, as watched. You should then have the title on Trakt website, showing as viewed twice.
Go in to your History (Trakt Website) and remove one of them. You must do this within the History section and not within the movie details. Remove by clicking the tick (watched) and the next time Moviebase syncs, it should be fine.
This will only resolve the issue for that title, so you’ll need to do this with each title which has the problem. It seems to be every movie I watch, a pain…
[Update] Well! This did work but not anymore…