So this hen is about 4 years of age and it's in route to become mostly white right? and While she had a white flight feather she just started changing color right?
I am not sure what it is, but she should not turn splotchy white(in route to becoming white) after her daughter(at least in age) she should have had that change many months before her doughter.
I have.... The thing is. She is the mother of the color changing hen right? So she is older right? I don't think it's piebald. since three hens are changing colors at about the same time. I believe it could be something nutritional or something external.