What do you mean, you don’t know if there’s a traitor? Someone clearly just sabotaged you, intentionally, in such a way as to make it difficult to tell who it was. And tenji, who was apparently the only one who noticed, would be even more suspicious. “I noticed your plan had a pretty big flaw, but i didn’t say anything because you seemed so confident”? Bitch if you noticed and didn’t say anything it’s because you wanted to exploit the oversight. Here i can forgive them for not thinking clearly, but tenji outed himself in a huge way there, and with the way the chapter ended, yuuichi might have actually caught it.