Is that pullet laying? Once my pullets start laying they join in the pecking order and move onto the main roosts at night. Until then they are not allowed onto the main roosts by the adults. You can have exceptions so nothing is "always" but mine have been pretty consistent about this. My coop is much larger and I have more chickens so it is not exactly your situation.
When they are going through this, I don't care where my chickens sleep as long as it is predator safe and not in the nests. If you consider your run predator proof I'd probably let her sleep wherever she wants to until she matures enough to force her way into the pecking order. If it is not predator safe I'd want to close that door.
That is a tiny coop. I don't believe in square feet per chicken as much as can they get away from the others if they need to. If an immature chicken invades their personal space my adults are likely to peck it or attack it, otherwise they generally leave them alone. In that coop the pullet probably cannot avoid invading personal space if the door is closed. That makes it hard and is a big reason you need more room during integration than once they are fully integrated.
Does this happen when they are separated or only when the pullet is very close to that hen?
I don't have any great suggestions other than try to give her as much room away from the other two as you can and see what happens.