That's because
If Orin does not kidnap Yenna, she poses as a camp member claiming Yenna is the doppleganger and urging you to kill her. If you don't pass the DC check and notice something is off, Orin will slit Yenna's throat in her disguise and then laugh and reveal herself pleased with tricking you into killing Yenna. I assume that the game treats it so that if your party member gets abducted in the sewers, Orin as doppleganger already killed and disposed of Yenna. Or it could just be a bug.
I see. That makes sense. Not in "that actually make sense" kind of way, but "reactivity doesn't keep up again" kind of way.
I was also meet "Laez" in sewers urging me to kill Yenna. The issue is, of course, that without Yenna being there Laez have no way of killing her in the moment, if you decided not to do it. In the camp no one had anything to say - not about disapperance of Laez nor the girl. Cat has nothing to say neither.
