Board Thread:General Discussion/@comment-32182236-20190721003717/@comment-26006155-20200114142146

I mean that in Hard Mode, we play as Sans, who isn't possessed by anyone, before he travels back in time to create the Beta Timeline.

Whatever happened that caused Chara's fate to entertwine with Sans' hadn't happened yet. So she wouldn't have access to his memories of this particular adventure before the timeline changed. Which is why she thinks things are going wrong.