I went through the same thing last year!
https://www.backyardchickens.com/t/909157/red-rum-i-mean-red-bum
I now have a completely different hen with the same issue. Seems like one hen is picked out to be pecked out around this time of year. I tried everything before I found out someone (????) is a feather picker & I put blu-kote, pine tar, or Vaseline on the bald spot.
Did the blu-kote stop the pecking or make it worse. I have it but have never used it for fear the blue color would just encourage them to peck at it.