The Adventures of Huckleberry Finn by Mark Twain
Did you know that Elijah Wood once played Huck Finn in 1993? Neither did I, but that’s beside the point. The point is, The Adventures of Huckleberry Finn ends not with Huck realizing that Jim is a human being worthy of happiness and respect rather than just a former slave, but with Huck saying "SEE YA" and noping right the hell out of Mississippi.
I'm not here to argue whether or not this total and complete dearth of anything resembling character development is believable for a teenage boy growing up in the antebellum South. I'm just saying it's a letdown either way.