Individual category and tag aggregate pages are displaying the latest posts only for admin users.
For an unauthenticated user, they are only seeing posts from Dec. 2013 or older. I was not admin on this blog until this week, so I don’t know what updates were made around December of last year – if any.
The theme is custom. I am guessing that the archive.php file was either modified improperly or that a plugin is causing an issue.