The writers of Avengers: Endgame and Infinity War have revealed that Smart Hulk originally made his debut in Infinity War, but a late change saw him cut from the film and reserved for Endgame.
As reported by Gamespot, screenwriter Steve McFeely explained in the Avengers: Endgame audio commentary that the merging of Bruce Banner and the Hulk was supposed to happen in Avengers: Infinity War.
“The whole arc of the story was Banner and Hulk were not getting along. Hulk wouldn’t come out to help him. And at his hour of greatest need