Thanks for the explanation.Getting a ragebait vibe from this but just in case you're serious, I'm pretty confident this was a flashback chapter and he just doesn't yet realize that it was Nanjou-san in the flashback. It's a typical "he was always in love with her and just didn't know it" kinda scenario but to your point, when she changed her look and demeanor, he clearly doesn't recognize her.
Anyway I'm done yapping.