In the United States, teacher-student romances are met with strong condemnation. There was once a test video where a female teacher was seen at a restaurant with a younger male student. The moment the actress playing the teacher showed any intent to seduce the student, people stood up to denounce her and even called the police.
Joseph didn't know about Ren He's relationship with Yang Xi; he only knew Yang Xi was Ren He's student, nothing more. During their time at the Juilliard School of Music, Ren He and Yang Xi were very careful about this because they knew what kind of media narrative a teacher-student romance would spark in the States. Moreover, he didn't want to reveal his identity as Knight, so it was normal for Joseph to be in the dark.
Now seeing Yang Xi in Ren He's home, acting as if it was her own, Joseph naturally realized the two were in love, and what's more, it was a teacher-student affair!
