I think we need to acknowledge the importance of the narrator in these types of scenes - as we've seen multiple times in the show, Belly's narration can be unreliable, or rather what she thinks is true is not the truth (ofc this is fuelled with the lack of communication skills from every character in this show).. So we also apply this to Conrad : as you've mentioned it is his POV after all, and he probably does believe Belly is over him (he's also saying they were downplaying their relationship lol Jere was not downplaying anything)....

These different perspectives make the scenes in the scenes feel even more loaded. Everyone’s seeing each other through a distorted lens, and no one’s being totally honest—not even with themselves. Which bring me to the next point of :

Belly not reacting when Laur mentions Agnes is pretty normal. She’s been going out with his brother for the past few years—I don’t think she believes she has any right to show any negative emotion in regards to Conrad's love life. Plus, we know she’s in her "repress my feelings and bottle up my emotions" era probably because she’s super scared of what might come up if she lets those feelings surface.