Avengers 4: The Real Reason Hulk Has Been Acting So "Strange"
It's not about alien impostors...
We need to talk about Hulk. Specifically, we need to talk about the fundamental misunderstanding of his arc that has so far inspired TWO fan theories seeking to explain why he's acting weird.
Since the release of Avengers: Infinity War, we've seen suggestions that Hulk is either Loki in disguise or a Skrull (more recently) and has been in disguise since the events of Thor: Ragnarok. Which is why he's struggling with erectile dysfunc... sorry, Hulking out.
Both theories fundamentally state that Hulk isn't acting as he should be. That he's being weird - particularly in terms of Banner being able to call Hulk out whenever he wants to. Both have posited that the reason MUST be that it's not Banner at all but a stand-in who can change his appearance but not his powers (hence him being so easily beaten up by Thanos).
But that's just silly and most importantly, it completely misunderstands the narrative journey Hulk and Banner have been on since Ragnarok. Hell, since Age Of Ultron, actually, since that's where it really started. You know, the narrative journey that we were told by both Mark Ruffalo and the Russos that he would be going on for three films from Ragnarok onwards.
That fact seems to have been lost in the mix as fans seek to find a zebra answer to a horse question. In other words, we all need to stop looking for outrageous answers when all of the facts are laid out in front of us already.
The real reason Hulk isn't acting the same is because he is not the same. After hints establishing that he and Banner were already wrestling for control "of the wheel", Ragnarok saw Hulk necessarily thrust into the driving seat as he fled to space. Because The Grandmaster fed his Hulk persona with something he'd never encountered before - glory - he was validated enough to temporarily become the primary identity in Banner's body. So Banner lost control and couldn't return for two years.
Eventually, thanks to Thor's meddling, Banner returned and Hulk was pushed back, only to return for the final fight and to presumably stay around as the Asgardians fled their planet (since he's on the bridge of Thor's ship at the end of the movie). From there he met Thanos and had his big green backside handed to him, once again tasting something he hadn't really before - a resounding, almost casual defeat.
At this stage, Hulk's mentality has been conditioned by him being in control for two years and by everything else that came before it. He realises that Banner has been using him - calling him out when faced by a threat he cannot hope to deal with alone. He uses him as a weapon that is then immediately resheathed when he's no longer needed.
Now, contrast that dehumanising behaviour with his two years on Sakaar. There, he was a God, cherished for everything that Banner viewed as a distasteful necessity. And even more importantly, Hulk being allowed to blossom actually improved him as a character - he was able to speak, his intellect improved, he became more human (if a little stunted). That film essentially confirms that what Banner is doing is abusing Hulk through neglect. So it's no wonder him being beaten by Thanos inspires a moment of stark realisation.
When Hulk refuses to be used by Banner during Infinity War, he's striking out for his independence - or at least for fair treatment. He's sick of being used and this arc will lead towards the pair finding some sort of satisfying balance between the two identities (perhaps where Banner allows Hulk out when it's not just to fight things?). Because that's the single most important driving factor in the difference in his behaviour.
He's not an alien or an impostor, he's just unappreciated, which is enough to drive anyone to extreme behaviour.