KAKURENBO: HIDE AND SEEK

Among the ruins of the city, there is a game only children can play. It is called “Otokoyo”; a game of hide-and-seek. But whenever children play this game, one by one they would disappear into the blinds of the buildings… The story begins when a boy enters the world of Otokoyo, to find his missing sister. — Description from AniList community (anilist.co).