It's good that the off behavior and whole setting himself on fire is a clear indicator for Yuya to be on his guard. He leaps to action as the flames come after him, throwing himself sideways in a panicked dodge before throwing his shield up to defend himself. Rin definitely shouldn't just be attacking him out of the blue.

"Rin, were you captured too?!" That makes the most sense, right? Like Gongenzaka, he had copies that attacked them too. He looks around, trying to spot another one, or the prism for confirmation.