I found the WP Admin UI Guide plugin to be causing this with taxonomy-related menu items all over WordPress. Your plugin is not at fault. Consider this resolved/invalid.
While we are at it, do we really need to get those thumbnails in get_category_hierarchical_terms()? Commenting that line out reduced the media library page load time from 10s to 2s, and so far nothing seems to be broken by that change.
This doesn’t work with user taxonomies if these are not managed similarly. So the question still stands: how to get posts that have NO association with any of the terms of particular taxonomy?
