Hi . No it is not a feed problem...Im working on an austrlorps project right now.all the breeding birds are blue ribbon or BOB., then breeding back out into origional foundation stock(nother breed). I dont have any white feathers . Sometimes an injury can damage the feather folicle and feather will come out white, Sometimes accidental inbreeding too much ,might buy from somone outside that are related and you dont know it. Have you ever seen a herd of piebald and white deer? they get into pockets and cant outbreed this happens. or at some point a white bird was introduced way back and genetics being what they are crop up.
have to wonder maybe somone was working on a lavender back a bit. and probably because the blacks out of it were pretty exceptional they kept breeding them.
It is a serious fault and DQ but...seems to me if they are real good birds you might be able to correct it and it would be worth it.
Can you take some pictures of the birds. A freind of mine had some super nice hens that had folicle damage from a favorite hole in the fence they liked to stick thier heads throu to be nosey , The damage was permanent. They dont produce any white cause it isnt hereditary. they just have white feathers around head and neck. If they are great in every way , just dont show them, work on improving that problem while keeping thier very good traits.it will crop up now and again. but those cannot be shown, This breed in america is seriously lacking the great ones. So they tend to inbreed the real good ones. Just a lack of top stock causes it.
Love to see your birds.