Is Mildred laying eggs?
She may just be mean. You never know. The damage could be from Mildred's nails.
Try separating her out and see what happens, but the injured hen needs to be healed and feathering in before you put her with the others, you don't want them to pick at her wounds.
I've had a few hens mount others for a while after I removed a rooster from their flock. Show of dominance, but they stopped a couple of weeks after and all settled down..
I did have 1 hen that was absolutely nutty running around and mating
everyone including the rooster (Yes, she'd tackle him and give it go!

). Anyway, I culled her. I did take a look inside and her reproductive system was shrunken and necrotic looking. Poor thing.