Hi! Chiming in to add that I’ve had two with this issue and have not figured out what’s going on. One of them has passed, due to internal issues unfortunately. But I still have one hen who has had a bare underside since spring or early summer, I believe. Originally I thought she had plucked the feathers, as she does like to to broody. I have checked for mites and sprayed down my coop and roosts with Elector PSP once this year. My vet had wondered if red mites (?) were the cause — that feed at night and live in crevices and whatnot. My hen still has the bare skin and it’s pretty red. The vet recommended I rub some SSD cream on the red areas for a few days, which I luckily had for my horse. And that did seem to help with the redness.