This, to the best of my knowledge, is actually [meta-tag:by-design]. In [this](http://meta.stackexchange.com/questions/43019/how-do-comment-replies-work) MSO thread, it says that as you could only be replying to the person that asked the question (in your example) then the @Username is stripped out automatically. If you can fully understand that answer in its entirety I would be more than impressed as it's not exactly clear.