Adastra Spoilers Below
That's what I was thinking. It makes perfect sense why Amicus would appear "stiff" and "intimidating" when in the presence of anyone but his human lover (especially the Khemians); not only is he the emperor of an intergalactic empire but he's also the face of the weakest sibling in the Galaxias. It's been 3 years since the events of Adastra and after the horrors Amicus witnessed (his imprisonment, attempted assassination on his brother, Cato's coup, the riots, death of Marco and Alexios's infiltration in the royal family) I find it completely reasonable to assume that he's matured since then now that he's taken on the responsibility of being the progressive emperor to an otherwise traditionalist empire. But that's just my 2 cents.