Dazai probably left him after he nullified Chuuya's corruption. You found him on the ground, Mori sent you to get Chuuya. Chuuya was sleeping, tired and injured. Then he woke up.

"Huh… Oh… What's going on? Where… What happened?"

He didn't remember anything about how he used corruption, but Chuuya always felt weird afterwards when people made him aware of what had happened. He hated how much corruption and the Arahabaki distanced him from his humanity.