Whenever I am responding by using "add a comment" I always begin with @username. Here are what I observed and am curious for their explanations:
(1) Sometimes the system prompts the complete username after I just type in the first few letters, but sometimes the system does nothing.
(2) Sometimes the @username disappears after I post it, presumably indicating that it has been acknowledged by the system, but in some other times the username stays.
Thank you for your explanation.
The main underlying reason for what you see is that in cases were explicit notification is not necessary as it would happen automatically (notifying OP of a post, or OP replying to a unique commenter), it is stripped or at least not supported via autocomplete.
Moreover, only usernames that actually would result in a notification are autocompleted, e.g., if you would try to notify questioner in a comment on an answer (where they have not commented) it would not work and there is no autocomple for this reason.
There are some more subtle aspects to this (e.g., editors of a post can be comment-notified but there is no autocomplete) and for all the details it is best to see the How do comment @replies work? on meta.se as already mentioned in a comment.
@replieson Über-Meta $\endgroup$ – Daniel Fischer♦ Jan 21 '15 at 14:24