I think it's bcs u forgot that this is an isekai. She is not the owner of the body. Bai yu and yan an loved each other b4 the fmc took over the body, which is why the fmc's heart beats faster when he's around. Also, now that the empress seems to rlly like bai yu, they can't rlly have a relationship even if he doesn't officially belong to the empress bcs it would mean angering the empress. B4, yan an didn't rlly know why her heart was beating around him but she's recently realized that she likes him, mostly bcs the former owner of the body rlly loved him since the heart belongs to the former owner. This is also why fei yue is scared of bai yu since he knows how much yan an loved bai yu and how she would rather much be married to bai yu than him.