I received a comment to this answer on tex.sx, and "@Hendrik." was used to notify me:
According to How do comment @replies work?, the trailing dot shouldn't have mattered, and I should have gotten a notification of the comment. Instead, a strange thing happened: I didn't get a "red-dot-notification" in my inbox (in fact, there's still nothing about this comment in my inbox), but when I hit the (non-lit) envelope, I found the reply.
Maybe some recent change to the comment-reply feature caused this?