OP has Sezul listed as one of the NPCs they've played and replayed.
Tataru was listed in patch notes 2.55 as not counting towards the achievement (probably because she's temporary/missable), so I believe that one is deliberate and wouldn't have affected your count either way.

If I had to guess, it's the Triple Triad Master not counting towards the achievement. But that could also be deliberate since, if I recall, you can only challenge them by playing a tutorial match, rather than a normal one.