It was poor rotations imo. Thibs played Brunson and Randle the entire third quarter. By the end of it, they were tired and slow to close out or rotate. That helped Bucks' shooters get open and start to heat up. Then, in order to rest Brunson and Randle, Thibs had to rely on Evan Fournier and Isaiah Hartenstein for some extra time while they were both struggling. Hart came off fast for Mitch, but Fournier stayed out there way longer than he should have...which got exacerbated when Grimes waited at the scorers' table for almost 2 minutes because New York couldn't afford to burn a timeout and get him in.
The Bucks also made an in-game adjustment to go away from Giannis and let their shooters beat closeouts. It worked, but I'm not sure there's an obvious solution. If the Knicks left Giannis alone, he'd have done the damage himself instead. But it's worth debating whether New York should switch the scheme and limit three-pointers when they've got a double-digit lead to protect with dwindling time.