Definitely not a bug. It's clear that the girl is not Hannah when the narrator says in Chapter 7: "It’s another redheaded woman entirely. Though you’re certain you’ve never met her before, you still feel as though you know her." Then in Chapter 8, "Your vision suddenly blurs. As it clears up, it’s no longer Hannah that is bent over in front of you. You see the body of the woman who you saw while having sex with Hannah last night." It's the same woman, but from a different … err … POV.When you have the threesome scene, there is a part where mc says he sees the girl when having sex with hannah. i dont recall seing that scene after multiple run. is that a bug ?
That woman will become known in the future.