I have not come across this type of feather change in chickens, only in parrots. In parrots, problems like this often indicate Parrot Beak and Feather Disease, which is not zoonotic to chickens so it is not the case. Other reasons for feather discoloration and poor structure include liver or kidney problems, medication, malnutrition, and over-abundance of certain vitamin supplements. Since you are seeing this only in one chicken in your flock of 50+ I don't have a good reason to suspect that she's been getting too much of any particular vitamin. Typically, malnutrition problems manifest in brittle feathers, or feathers with stress bars, etc. Her feathers are not well formed but seem okay otherwise. A bit puzzling!!
I do know that Vitamin D3 deficiency can cause feather issues. As I assume that she is getting the same feed as the rest of your healthy flock, perhaps there is something going on in her body that is preventing the uptake/absorption of this important vitamin. Do you know if she has started laying yet? D3 usually shows as other symptoms as well, and if she seems otherwise healthy, I am not really sure if it would be a D3 deficiency. I've also read that this tends to darken feathers, not lighten them. Still, it is worth me mentioning, just in case.
http://www.poultrynews.com/New/Diseases/Merks/207019.htm
Pantothenic Acid deficiency can also cause ruffled, unusual feathering in chicks, but I think you might have seen this before her 21 weeks along with other signs! Maybe not? I am not an expert. It might be worth exploring.
http://www.poultrynews.com/New/Diseases/Merks/207025.htm
Folic Acid deficiency shows up as poor feather condition and depigmentation (white, usually). The feathers usually become brittle though. Still, it is something to consider!
More info:
http://www.poultrynews.com/New/Diseases/Merks/207028.htm
There are other sources of info online too, if you google "____ deficiency chickens" you will get results.
Maybe it's none of these.. or even a combo of more than one. She may be having trouble absorbing vital vitamins from her diet for whatever reason-- much like people with osteoporosis that can eat calcium until the cows come home and not benefit from it, she might be eating the vitamins she needs, but cannot absorb them.
I regret that I don't know the best course of action to help her, since a blood test might be out of the question for you? You can attempt to supplement her, or just observe her general health and see if she does well despite the strange feathering. I'd just be cautious though, as strange feathering is usually (not always, but often) a sign of other health problems.
