It could be:
1. Just like beware of the Villainess (manhwa) story. There might be someone who controls the world and will forcefully try to "fix" the storyline. And since she didn't know yet if that kind of being exists here, her doubt are acceptable.
2. If we follow Miss not-so-sidekick (manhwa) story, you can see that even when Latte appear and somehow messing up the story here and there (albeit not too much), in the main storyline only Arwin who were able to fully break through the ibelin's charm, hence derailed from the story. In here, again, she didn't know whether the heroine's charm is omnipotent.
3. The heroine is a reincarnated person as well (a lot of Villainess manga). This might results her wanting the ML too.
4. The author doesn't really have a reason. Well, this one happens a lot too.. XD