Seo tends to make heroines to leave for a while ... happened in Kimi, happened in Suzuka and happened here.
Fuuka was just stupid, he actually did have a heroine rejecting the male lead in Suzuka when Suzuka was overseas with nobody making drama, if he say killed Suzuka the reaction would be simular ... besides he just Isekai Fuuka for the shock value with no idea were to go next and it shown.
I know lots of (most?) people think that way about Fuuka, but I personally only started reading Seo's manga whilst Hitman was publishing. On top of that, I've been involved with music my entire life. So being able to binge read the manga greatly improved my experience, as I'm pretty sure I wouldn't have enjoyed it as much reading it weekly. And having been surrounded by music forever made even the insignificant moments in the manga feel like I was back home.
Obviously this comes down to my personal experiences matching Fuuka's tone almost perfectly, and I can't blame anyone who isn't in the same boat. I honestly think how Eba left in Kimi is exponentially more egregious than how Fuuka left, but that's coming from someone who loves Fuuka and is just a little biased.
Either way, I hope we can agree that how Ouka left and just returned is the best that Seo's done out of this, Suzuka, KNIM and Fuuka.
I don't really count Hitman. It feels like the black sheep of the family, and it's the MC who leaves anyway.