There’s also the new Millenium Actress dub.It's quite possible that the film is HELLO WORLD, from director Tomohiko Itō (Sword Art Online: Ordinal Scale). Anime Limited have already ruled out Fate/Stay Night [Heaven's Feel] II. Lost Butterfly, Haikara-San: Here Comes Miss Modern Part 2, KonoSuba: God's Blessing on this Wonderful World! Crimson Legend, and Rascal Does Not Dream of a Dreaming Girl. There's also the film Her Blue Sky but the timing may prevent it from being the mystery film.