This seems to be a weird bug and not present at all times.
Also it seems that XnvieMP does not correctly handle assigned categories and ratings when moving.
Here is what happened:
1. Move file to other folder via context menu.
2. When prompted that source cannot be removed skip.
3. Check in destination folder, chances are that there is the file that should have been moved without any ratings or categories now.
4. Now move the original file again and choose overwrite.
5. Check destination folder. Chances are that the file still has no categories or ratings assigned althoug it has been overwritten thus the old file is no longer present.
6. Move the file back to old source folder, in that folder chances are that the file is shown with ratings and categories again.
This behavior is somewhat shocking. It means that somehow XnviewMP stores ratings and categories even after a file has been moved. So they depend on the folder a file is in and not on the file itself. This can lead to totally wrong display on category results or even erroneous deletion of files.
Edit: It seems that similar issues have been reported with previous versions:
viewtopic.php?f=82&t=34069&hilit=rating#p135796