Rewatching that scene from “Poké Ball Peril” made me think about the difference in Ash’s reaction when the same thing is suggested ~75 episodes apart.
Daisy assumes that Ash is Misty’s boyfriend in “The Water Flower of Cerulean City”, episode 7. Episode seven, so since Misty started following Ash at the end of episode 2 they’ve only spent four full episodes together at this point. They don’t know each other much, they’ve hardly ever acted civilly towards each other, I wouldn’t even say they’re friends–Ash has hardly seen anything of Misty as something other than the weird girl who’s just following him because of her bike. I really don’t think he’s had any thoughts or feelings of the romantic sort for her at this point, even subconsciously, and his reaction is in line with that: he doesn’t blush (at least from what we can see of his face), doesn’t get angry, he just falls to the floor anime style. Overall he seems more in disbelief that someone could think such a thing than actively bothered by it.
Yet when Team Rocket insinuates pretty much the same thing in “Poké Ball Peril”, episode 83, he’s definitely bothered by it. He blushes, immediately denies, sweats (look at his brow in the last cap), gets Pikachu to shock them to shut them up. And this time we’re on episode eighty-three–he and Misty have actually spent a lot of time together. A lot has changed in the meantime: they’ve become friends, the bike is almost forgotten, they’ve grown comfortable with each other, they’ve helped each other and seen each other’s strengths and weaknesses. They’ve gotten to know each other properly.
And judging by that very different reaction, something else about the way he feels about her has changed too.