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Tagging people in comments doesn't seem to work for me, at least not always. I just tagged someone in a comment I made (not the first comment on the answer, if that matters), and the "@xxx" part wasn't included in my comment.
I don't know how to put this, but sometimes when I tag people there pops up a "suggestion" (not clickable). It didn't in the above situation, for example, if that matters (and if you know what I mean...).