I will definitely keep track of the ones I breed just in case it pops up again. It is odd that none other have it, but since there isn't a history of the leakage in the bloodline, could have been a mutation in that one bird? Things like that can happen. I don't know, just throwing out ideas.
Spontaneous mutations can happen, but they're not that common. I did some studying of spontaneous mutations, related to a deadly syndrome in Labrador Retrievers, tricuspid valve dysplasia. Some folks were not accepting this was a genetic problem at all, but the result of recurring identical spontaneous mutations. Uhm...nope. Spontaneous mutations just aren't that common, and the statistical probability of them all being alike is astronomical.
Of course, none of that has anything to do with this. lol It is more likely that this is a hidden gene that was exposed when just the right genetic combination occurred between the two lines. I'll just really need to watch for it and track it down.