As far as I know Feather Fixer is 18% protein and 3-4% calcium.
Long story on the marketing 'scam' of that feed.
What are you feeding now?
Chickens go thru 2-3 molts between hatch and about 5-6 months.
Hard to say if the missing feathers in this case are a molt or some picking due to confinement and the feed running out...I'd guess that latter.
As long as there's no wounds, I'd not worry about it unless it continues.
Feathers will not regrow unless they were fully plucked out, watch area for new pin feathers.