Because he's the emperor so his stance and behavior will change. But still, you still can see his goofy personality when he deals with Neferu so yes, he does change but not much, just more serious than before because now he is a emperor
Also they probably didn't want Amicus to be as cute as he was in Adastra since we aren't romancing him this time, it'll let us focus more on Neferu
Ya your probably right
