I keep looking over those photos and it looks to me like her feathers are getting rubbed off there. Do you have one of those feeders with just opening for the beak? I'm wondering if it's rubbing off her feathers, or if she is sticking her head where it doesn't belong, with the same result. Do you ever see her pecking at her own feathers? Have you checked her out top to bottom for lice/mites? She looks to be the same breed as the other in that color.
Do they all sit near each other on the roost? That it usually a prime time for another chicken to start pecking/plucking.