This movie is a vanity project by and for a woman with a savior complex who wanted to feel good about herself and didn't care enough to learn the first thing about the community she was depicting. That is the story of this movie, from beginning to end.
Bad enough that she took advantage of her position as a mentor and pressured an underaged actress into doing a performance that she didn't want to do, but the performance she was pressured into doing is an appallingly offensive caricature of autistic behaviors more akin to a playground bully's mocking than actual acting, and the way that the narrative is constructed and the way that many scenes are framed makes it very clear what Sia thinks of autistic people, and it ain't good.