add_rewrite_rule behaving strangely

So I’ve solved this problem, but I’m still scratching my head about the “solution”:

add_rewrite_rule('api/autho/?$','index.php?__api=1','top');

This version of the rewrite rule works for some reason. I’m guessing that there’s some sort of namespace conflict that I accidentally stumbled across. Maybe /auth/ is used somewhere else?