Everyone calling him a dumbass, but let's be honest; if you met someone from when you were in elementary school and they were completely different in looks and demeanor and actively tried to act like they don't know the person you're referring to, would you have any reason to believe they were that person?
Hell, I just ran into someone who was in my graduating class 15 years ago and, only after about 5 minutes of talking did he finally introduce himself because he recognized me (aside from a receding hairline, I look the same as when I graduated) and I still had a hard time recognizing him or even remembering what he looked like back then.
Our dude's confusion is not that far-fetched. Also, glad we got Mahiru there, because we need someone else reacting to how stupid Miya is being, or I feel this would be much more tiring to read.