(I haven't seen Season 3 lately, so forgive me if I don't get the sequence 100% correct, but this is beset recollection of how the Aquaman 2 stuff went down.)
The guys learn that Medellin and Aquaman 2 are set to shoot at the same time. Ari, E, and Vince try to haggle with Warner to shorten the shooting schedule, postpone it until after Medellin, anything. Alan tells them he can't do it, he understands "passion projects" are important, but the studio can't shorten the schedule as much as Vince needs it to be shortened and they can't afford to delay production because "we've got a ten million gallon water tank sitting out there." The guys try to get the Medellin folks to alter their schedule, but they can't shorten theirs either. Paul Haggis shows up at Vince's house late one night with a bottle of wine, says he can't imagine the project without Vince, and proposes a brainstorming session. Haggis and the guys, along with Saigon and his bro, sit around for hours, before finally coming up with a solution: they shoot part of the movie before Aquaman, part after. Haggis thinks he can make that work. All seems to be well.
Then Alan tells Vince that Warner can't let him do Aquaman - he's the golden boy of their franchise, and they don't want people seeing him snort a line of blow on every big screen in America. When questioned, Alan admits that he never seriously wanted to knock some days off the shooting schedule or help accommodate Medellin. He didn't think they could pull off both movies and thought he'd just give the appearance of being helpful. This duplicity pisses off Vince and he raises his demand on Aquaman to some ridiculous figure like $20 million. Alan gets pissed, Vince gets pissed, and Ari tries to make nice with everyone, but Vince gets fired off Aquaman and Alan all but promises to deep six Vince's career. Shortly thereafter, Alan buys the "I Wanna Be Sedated" script from Bob Ryan and ensures that Vince will never get to do that movie.
Alan Gray is a mean, vindictive human being.