I didn't realize this either until fairly recently when I'd gotten some juveniles and one had some flight feathers that had been plucked by other ducks. We're always learning. :hugs
I've upped the amount of protein even for my adults now, since it's getting much colder out there and they...
You may want to return to the 22% until they are at least eight weeks old. They are still growing and need a lot of protein to make those feathers, which is why sometimes they will pluck them to eat.
If there's no feather sticking out oddly, then I would leave it. It could have been one of the other ducklings (or even himself) that pulled at it.
What % protein is their food?