The ending of the film (spoilers ahead) is one of the most universally disliked aspects of Joker: Folie a Deux, a miserable end for Arthur Fleck and the alleged birth

One of Joker’s planned endings never happened because Christopher Nolan reportedly stepped in to veto it. Why Did Christopher Nolan Veto the Ending of Joker? According to a report in The Hollywood Reporter,

Nolan no longer works at Warner Bros, switching to Universal for Oppenheimer. This meant Joker director Todd Phillips was able to include it in the film’s sequel.