I think it is just part of the roaning. Faded black ? urine stains.
I live in a very hot very sunny area, and many black horses will sun bleach.Never heard of anything like that being part of roaning? But I’m open to learning if that can happen. Won’t be urine stains as I’ve given him multiple baths and it’s stayed the same. Could be faded black but I’ve never heard of that happening just in one spot and also I’ve only ever seen black bleached by the sun in the summer, and my horse’s leg stayed brown throughout the winter.