Its a flaw from intense inbreeding. I imagine it will work itself out as the variation becomes more popular and the breeding stock gets larger. As I think the fellow in the link above has done, used disparate specimens to breed, rather than fish purchased from a single batch at the fs like most amateur breeders do.
Yeah, I read that above. Line breeding isn't great for most animals. The traits you want to genetically fix often also get fixed with other undesirable traits. E.g. German Shepherd Dogs and hip dysplasia, Dairy cows and chronic lameness, Thoroughbred horses and hoof density problems.