Actually, Yuuya fails to answer his phone when invited to the summer festival in every single route except his. The somewhat disturbing implication was that he canonically dies every single time you aren't there and don't specifically rescue him. He does show up alive and well in Anghel's route and Sakuya's extended ending, but ... being an extended ending and Anghel's route, I'm not sure how much they count.

Oh, and now BBL, of course.

Also, even if he escaped and came back, that would be pretty gutsy of him to suddenly start working closely and collaborating with the doctor at every turn as soon as the dome goes up....