Casting shakeups happen quietly in Hollywood all the time, usually explained away with a vague nod to scheduling conflicts that no one questions too hard. That was exactly the story fans got when Paul Mescal exited Ridley Scott’s post-apocalyptic thriller ‘The Dog Stars’ last year, with reports at the time pointing to the actor’s commitment to Sam Mendes’ upcoming Beatles anthology as the reason for his departure.

That tidy explanation didn’t hold up for long once industry journalist Matt Belloni’s reporting for Puck surfaced this week. According to Belloni’s sources, Scott had privately grown unhappy with Mescal’s performance in ‘Gladiator II,’ reportedly feeling the...