One of my 2013 sites has an MMS system set up for tagging images in a document library. It was functioning perfectly for months while one of our co-ops was working to tag everything in the library. But just recently the tags stopped showing up when viewing/editing any document. The weird thing is they are still searchable, but when you look at the document the tags field is blank. It still registers that it was edited by the user, and when editing the document you can add tags, but they don't show up after it is saved.
Another odd part about it is that when you search for a document by one of its tags, it gives the message "Your search returned no results. Some files might be hidden. [Include] these in your search." Once you click "Include" the results appear normally. This only affects searching for tags, not for any other property of the documents. I'm assuming this is related, but I don't know where to start troubleshooting it.