Spoiler Thread: Avengers Infinity War

They probably filmed that Trailer first thing so they could have for promo. They didn't want Hulk to be completely jobbed so they changed him to not come out and they inserted the Hulk Buster, which is why the CGI for Bruce in the buster looked so terrible at the end of the movie. The CGI of Bruce's head in the buster was jarring and was the only part of the movie that removed my immersion from the film.