Max, have you considered feather mites? If not that, my guess is a combination of nutrition and genetics.
Not feather mites... After the ordeal I just went through, I probably have the most clean and disease free birds on the planet right now.
I haven't changed the feed much so I'm leaning towards genetics. As my lines have gotten more concentrated, It has shown up a little more frequently. Great... something else to cull for.
It's disappointing when you spot an exceptional bird and open the primaries to find this condition.