From what I have read/heard from long term breeders, There is a genetic predisposition to angel wing in sebastopols.In fact, several years ago when I just started with them, just about all mine had angel wing. I also lived in a place where they could not graze, only eat poultry food .
It is rare for wild geese to have angel wing if they graze exclusively. When they are fed by humans, some can develop it as youngsters.The higher protein in the feed is thought to contribute to faster wing growth resulting in wing abnormalities.
Over the last 2 years, I have rehomed just about all my sebbies that I had hatched from
ebay/eggbid eggs. I have started over with Holderreads stock. I had been quite anxious to have sebastopols when I started and felt that hatching was the most economical way to go. Between poor hatches from shipped eggs and less than wonderful feathering in the geese I had hatched, I was unhappy with my stock.
Just about all my current birds are gorgeous. This year I am planning to keep more for breeding purposes. The last 2 years, most were sold as goslings to purchase better quality whites and some colored sebastopols.I also lost some due to predators and reproductive issues.
I will add that I also have changed the way mine eat. They get poultry food in the am and pm, but not so much that they are not hungry and graze much of the day on our grass here.I have found the wings are not as prone to angel wing issues. My giant gray dewlaps have never had any angel wing issues, years ago when I lived elsewhere or here. I see a definate difference between the two breeds for tendency towards the problem.
If you get youngsters, be alert for wing issues and if they develop, you can correct them in your birds if you catch them in time. There are sites that will tell you how to wrap the wing while it is still growing.