Responses
That would actually make a lot of sense, but right after grey haired guy went back home with the time machine, we could see black haired guy was asking the teacher about it and trying to figure out how to get him back/go to him.
So I think he really went back to the wrong timeline
I don't think it's a different timeline like everyone is saying? Like I think it's still the same timeline but he went way back again to the point where black haired guy still didnt know who he is. He simply went back to the first time they meet each other not after he left to go back to the future. Kinda like a game save point. You accidentally deleted your progress so you have to start all over from the very beginning instead of continuing where you last left off (because you can't do that since the save game is lost). As to why black haired dude was acting different from the first time they met, well, a person and their responses to certain occurrences or circumstances are not an unchanging consistency so their reactions to something may differ given the different instances even if the situation remains the same. Even in a probability estimation the unlikely divergence may occur. In the case of repeatedly going back in time to confront someone, a person when approached may not respond in the same manner as they did in the previous instance so that explains it.