So the rules what apply to Mai and disappears are odd. Food she holds disappears but not people. I guess perception could be at play here. Easy to lose track of a piece of carrot, but much harder to lose track of a teenager all of a sudden. Though the more we learn of Mai's past the more concerning it is. Her mother basically inserted her in roles that were uncomfortable for a kid. Though like it is mentioned last episode, Mai def wants to make a comeback. It would tie in more to her wanting to be seen then the opposite to never be noticed.
Although Sakuta learning the information def put her on edge. Though that reflects on his experience with Kaede. When one sees others in a situation they were in, it is easy to empathize. It was mentioned in episode 1 of him being interested in Mai is a surprising since he has only shown an interest in Shoko. But who is Shoko? It's clear she saved him, but he went to the high school he did was because of her. Though to find she never went there is even more odd.
Though more heartbreak for Mai. Her mom not noticing her was heartbreak. Even if you have a bad relationship with her parents, there could be ways to repair the relationship. Given she doesn't know Mai's existence, just hurts her. After all, what Mai planned for her comeback most likely had to involve her mom. You can see it in Mai's face and Sakuta seeing it upsets him.
In his conversation with Futaba, they mention the school and that holds the key. It seems like a very viable clue given that for a popular icon like Mai, school is one area where they can be normal.