Why does this always happen?

that's the easiest way for plot to happen lmao

In this case, I think it's normal for FL to be wary or scared of her situation, considering that original Jubellian, although was a 'villainess', she was innocent. Although she loved Mikhail, she didn't plot to harm Mikhail's love interest, even when she hated her. The lack of communication between her and the duke made her believed that she was unloved until her death.