It is a little obvious, but Yuya's not terribly subtle about his emotions to begin with. Having a tail is a new experience, and he hasn't totally clued in the way he moves it to his emotions.

His face scrunches up as he stops in front of Lambert. "It's not my first change... the rest just aren't all visible."

Or easy to overlook, like his funny shadow. Of course, even those might be mild given the amount of magic he's learned, attuned to, or had stabbed through him.