AH :
XY was super protective of CX. She shielded his body with her body during two separate threats to CX's life (in chapter 6 and chapter 15). And when he married Xing Yue she felt like she lost him and almost left CX's side forever. CX had to threaten his own safety to force her back to his side (chapter 30). To someone who didn't have as much information about XY's feelings as the reader, those could look like the actions of someone who had deep romantic feelings for CX and was jealous and hurt as a result. But XY's feelings for CX were platonic.
TSJ thought CX was the same, and that he was being protective of XY as a brother figure who deeply cared about her. That CX was being cold to TSJ and extra protective of XY because he was worried that TSJ would cause her even more pain after already causing her so much heartbreak.
In the drama at least, I think the problem is that TSJ just doesn't pay attention to much around him, except XY. I think I've said this here before, but he strikes me as one of those smart (and rich) people who thinks he is smart enough to deal with any situation, so he doesn't always pay attention.
Whereas XL is always paying attention to other people and their environment. We actually see him watching people repeatedly. Like in that house party scene when FFB sitting with FFYY and TSH, while across the room Jing was playing Go with Feng Long, and CX and XY came in. Then XY demanded he come out to walk with her. There's a great exchange where FFB looks at FFYY as she pours a drink for TSH. Then he looks at TSH. And then looks at Jing across the room, and you can see him filing all that away.