@Siikalahna - because, although she's finally noticed the deviation from the plot of the novel, she thinks she's still fated to be divorced regardless because Cesar doesn't love her. Basically, the skein of her life was tied long ago, no matter what choice she ultimately makes. And because she's not selfish and as oblivious as the original Canaria, she's just making small choices that she thinks will have no bearing in the end.
Let me guess, the prince and the MC will be speaking, something happens and she trips on him or something, then her future husband will walk in, thinking they're getting intimate.
@Emperor_Late and @Wilhelm_Stenvall I agree with you both. Her husband lovingly told her to stay there since she said he could go, but now she leaves. Just have that feeling something will go wrong