I'm trying to address a comment to the person who answered my question using @name. This is similar to what was described in this thread, except in my case, I'm the original poster, and I'm addressing only one person in my comment. In other cases, this works fine for me--in fact, while typing @name in the comment field, the user name usually pops up above what I'm typing as an auto-complete. In this case, there's no auto-complete, and after I submit the comment, the @name is stripped out. One thing that's different about this username is that his last name, which is German, contains an umlaut over the letter "u". Could the special character be causing a problem? Or, is this a feature that the user can turn off, so that he receives no notification of comments that are addressed to him?
The behavior is expected, as you are commenting on Marcus' answer, the OP in this case is the answerer.
In general 'OP' is used for who posted the question, but in the case of comments, the author of the commented post gets notified of all comments and that's why there is no need to tag him. (sorry for the repetition)