i noticed that too but the story is good so ill just ignore the inconsistency
she was a villainous her first life and then reincarnated to the modern world. she died then came back to her first life.
She was reincarnated from the modern world. The old Cayena died from the poison. Since the new Cayena knew the flow of the story (that's why she was able to predict and made movements to reach her goal) and somehow got the previous memories of old Cayena.
I don't know if it will answer your question tho . Haha!
I was wondering, isn't she traveled to the novel? But why the progress of the story seems like she return to the past?