In this answer, suddenly I can't notify Ben Bitdiddle, typing "@B" won't show the autocomplete box. I have tried write "@Ben Bitdiddle" manual but after I enter, this notify words disappear (you can see yourself in this latest comment. Notify other people in other comment boxes on that same question still function. Refresh the page won't solve the problem. I use the latest Firefox version in Windoes 8.1.
This user is the one who posted this answer, so you won't see autocomplete for his name (and you actually don't need to use @replies to notify him; he'll be notified anyway).
Moreover, if you type his whole name, it will be removed once your comment is submitted.
Since you are the only other person who has commented on his answer and he is the one that posted the answer, the @-mention at the beginning is not needed. It is clear from context that you are talking to Ben because he is the owner and the only other person talking.
Since the owner of a post is always notified of new comments anyways, it is safe for the system to drop the username at the beginning. The conversation is not affected in any way.