I tried to tag a user in a comment replying to his previous comment. Usually if I type @UserNa.... the name will appear in a pop-up field. But when I tried to tag one user, the name didn't pop up. When I saved the comment, the tag vanished.
The software tries to avoid @name syntax when it is not needed.
Presumably you were in one of these two cases:
You were the original poster and there was only one other user that had commented.
You were addressing the original poster of the post (and there weren't others commenting already).
In these cases, the other user is notified no matter what and the @name is not needed. Thus it is not proposed and even suppressed when the comment is submitted.