“Joker: Folie à Deux” is the No. 1 movie at the box office, but it might not be destined for a happy ending.In a turn of events that only Arthur Fleck would find funny, the follow-up to Todd Phillips’ 2019 origin story about the Batman villain opened in theaters nationwide this weekend to a muted $40 million, according to studio estimates Sunday, less than half that of its predecessor.The collapse was swift and has many in the industry wondering: How did the highly anticipated sequel to an Oscar-winning, billion-dollar film with the same creative team go wrong?Just three weeks ago, tracking services pegged the movie for a $70 million debut, which would still have been down a fair amount from “Joker’s” record-breaking $96.2 million launch in Oct.