from my reading of what indigenous people have to say, it’s a variety of factors. one is that Klune treats this as like an “Easter egg” so he doesn’t use it to raise awareness of residential schools or the atrocities committed to indigenous people, he doesn’t use it to lift indigenous peoples’ voices about their experiences, and there’s no evidence he’s actually done anything concrete to help, whether monetarily or just raising awareness
but the other thing is just how he went about this (keeping in mind he’s explicitly stated that Canadian residential schools inspired this story), where the schoolchildren (who are supposed to be indigenous children) are depicted as inhuman monsters that the nice white man has to save through the power of love and kindness which feels a little insensitive and white-savior-y
and again, from what I remember, there was no author’s not explaining where he got his inspiration or leading readers to places where they can learn more and/or help. tl;dr: books about the holocaust are usually more tactfully done and less exploitative, and are more criticized when they aren’t