In She-Hulk: Attorney at Law, Bruce explains to his cousin Jen Walters (Tatiana Maslany) that he designed a special Hulk Inhibitor Device, which he created specifically for transforming back to his human form. This is further confirmed as the exact reason why Banner appeared human in Shang-Chi when it's revealed that She-Hulk occurs around the same time as Abomination and Wong's cage fight. By the end of the film, Banner gave himself over to Hulk in order to try and defeat Hela, and Hulk was back in control at the time of Infinity War, but as Russo points out he was being used as a wrecking ball yet again.
The planet Sakaar is not safe and calm enough for Hulk to change into Banner. It's an environment with harsh condition near to death scenarios. Hulk and Banner are already separated minds.
This version of Hulk displays more intelligence and speaks with more complex sentences. For that reason, Banner can't remember what happened in the last 2 years. It was meant to be part of a bigger plot for Hulk/Banner which concluded on Smart Hulk busting out of the Hulk Buster during the Wakanda battle in Infinity War.
Then the Russos dropped the ball at last minute and deleted that from the movie. While Thor: Ragnarok raised the possibility that, because Hulk had been in control for so long, another transformation might permanently trap Banner and leave Hulk on the outside forever, Infinity. Early MCU Films In early films like The Avengers, Banner and Hulk were presented almost like separate characters inhabiting the same physical form.
Banner would change involuntarily into the destructive brute Hulk when angry, with little control or memory of events after transforming back. It was also claimed that Banner will once again have trouble transforming back into Hulk. It's possible both this and Firpo's comments are true and Banner will change back for a variety of reasons, helping his cousin along the way, though it's worth noting that the writer admits he doesn't know the details for a fact.
Throughout the movie, Banner tries desperately to bring Hulk out, but the green guy refuses. But it's not because the transformation mechanism isn't working. When Smart Hulk debuted in Avengers: Endgame, it didn't seem like it was a temporary state where he can easily go back and forth.
It's worth noting that the 2019 Joe and Anthony Russo-directed blockbuster didn't get into specifics about what went on in the process of birthing the hybrid character. So, naturally, there's a lot of questions about it. Bruce's appearance in Shang.