First off all, what does popularity have to do with anything? The important thing is whether or not it has the capacity to produce a drawing this level. Moreover, saying that it only wasn't as developed back then as it is now doesn't even contradict what I'm saying. Plus I'm standing on the fact that yes the art is not ai generated. I have been reading multiple stories with this artstyle for years now. The part you are talking about is most likely the work of the cleaner or the translator like the previous comment mentioned, not the artist that you disrespected with your minimal level of knowledge and observation skills (=・ω・=)

the first story the art is literally AI GENERATED
if you saw it in chapter 3 the uke’s head is literally ai generated like just because of that i’m dropping this wtff
NO WONDER THE ART WAS SO SHITTY BOOTY IT WAS ALL GENERATED BY SHITTY AI SLOP