Quoting myself from here:
The autocompleter only autocompletes actual commentors. We decided to not autocomplete editors, because
a) it's a very rarely used feature, b) it'd be quite some work to make this work nice performance-wise, and c) it would probably be more confusing then helpful ("Who the hell is this User12345 and why is he in my ...
Do the @reply get removed if the original comment is deleted or removed?
No. This is not possible.
What happened here—and I'm assuming you're talking about this post—is that the original comment and your reply were deleted by a moderator as they were too chatty and really not relevant.