Well, they did grow up. People, especially guys, can change drastically in 5 years. My bro grew nearly a foot taller and just looked different.
I would agree with you if it just looked like they matured but that’s not the case here. Go back and look through from volume 7 to recent chapters. The scene where Natsuki is chasing after the cat and yells “hey! Are you hurt?” (vol.10, ch.49) doesn’t even look like him. They don’t look bad, they just don’t look like themselves recently. I think it’s too dramatic to describe it as them being “yassified” but idk how else to explain what it is that bothers me
At first it didn’t really bother me but they look so different now. I really liked the way they looked in the earlier volumes too. I wouldn’t say the art style changed completely though, their character design has just drastically shifted