It should also be mentioned that people "disappear" all the time in Japan. About 100.000 person per year are signaled as "missing". Most of them are found or come back later, but some just "evaporate".
It's called "Jouhatsu" (a term meaning "evaporation"). It can be for a lot of reasons, for family reasons, for tax reasons, for debt reasons, a woman evaporating with her child, leaving the husband for DV reason (or others), suicide, etc...
The laws about privacy are really strong in Japan, so as long as no crime has been committed, police will be extremely reluctant to investigate.
So, I think there's nothing surprising about Aya's situation regarding the authorities for a Japanese reader. She disappeared, she's back, she's 21, no crime has been reported. Everyone moves on. It's not the police's problem.
That’s certainly true in general—the classic 1967 Japanese pseudo-documentary A Man Vanishes is about that very phenomenon. But most such cases are, as you suggest, men leaving their families, whole families running away from loansharks, or other grown people escaping bad situations like oppressive jobs, etc.
But as we saw, this was a missing middle-schooler who sparked a police search, one who has now returned physically unchanged and with (essentially) amnesia. Even given the cultural concept of jouhatsu, the specific situation would seem to warrant rather more than a “Huh, that’s weird. Well, welcome back!” reaction.
I'm honestly curious if they'll even address the disappearance with any kind of specifics, or if they'll just hand wave it away. Magical Realism is a decently popular genre in Japan, and the more I read this the more I feel it sort of fits into that genre?
No one (other than Koto I guess, but even then it feels more rooted in the effect the disappearance had on her, ie. her separation anxiety.) seems to be that pressed about the 'how' of her disappearance just yet. They're all just like, "Yeah, okay."
Anyways, super excited to see how this all plays out. This definitely feels more realized (fleshed out? realistic?) than the author's previous work. And I feel like I can understand where each character seems to be coming from when they respond to things happening around them.
Interestingly, I was giving the synopsis to my partner and she thought the story almost felt like the author was/is trying to sort through an event that has happened in their life.