This, to the best of my knowledge, is actually by-design. In thisthis 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.
replaced http://meta.stackexchange.com/ with https://meta.stackexchange.com/