I'm experiencing some occasional strange behavior with the "Att. Tags" admin page. Here's what's happening:
I import images with embedded IPTC keywords, and these keywords are being mapped to the attachment_tags taxonomy. If I edit the image outside of WordPress and delete certain embedded keywords, and then subsequently re-import the image into WordPress as a new image, the old keywords that were originally associated with the image (i.e. the keywords that were removed from the image) are still being associated with the image in the "Att. Tags" admin page. However, when you click on the number in the "Attachments" column to view the image(s) that supposedly still have these keywords attached, the keyword doesn't show up in the list of attachment tags associated with the image. It's almost as if the "Att. Tags" admin page is maintaining term associations that are no longer there. Perhaps when an image is deleted from the Media Library in WordPress, MLA is not removing or updating the associated media items that are supposed to have this keyword? Even this doesn't make sense unless MLA is mapping terms/tags to image based on something other than post ID?
The only way I've found to avoid this behavior is to delete the attachment tag term from MLA and WordPress, but this is not always feasible or desirable if the term/tag is still valid for other images in my Media Library.