Author can't make up their mind if they want Komine to be dense or not.
One moment he has the mind of a wise adult who knows exactly what to say to score points with Miyano, then the other he is a dense kid who cannot sense her obvious affection for him even though he is actively trying to get her to like him.
You might have forgotten since it has been so long, but Komine isn't exactly the most intelligent person around. He does have the significantly longer life experience than the other kids, but that's pretty much it. He was never a wise adult. He can say the right things, occasionally, because unlike little kids with immature emotions and personalities, he did live long enough to develop his own persona and had to make decisions for himself, as an adult. Adults aren't also held back in the same ways as little kids when dealing with the opposite gender (or older kids/adults). Many things that seem like huge deals in little kids' eyes, are meaningless for adults because they have much bigger worries in life. Right now Komine doesn't exactly have huge worries, but in his first life he had, and some of that adult worldview would remain.
Nevertheless, he's still stupid. Unlike a kid, he can call Miyano cute publicly, which would be a huge deal for a boy his apparent age, but still in the first chapters he went for it like an idiot calling Miyano cute nonstop every chance he got. It wasn't a big deal for him, but it was too big of a deal for Miyano.