Your picture looks identical to one I posted in early May.
When I got my 11 chicks at 5wks old, they had been in a small brooder for too long. They were scraping up their lower backs while going around the brooder. No feather picking (once out of that small brooder and into a HUGE coop and run), no mites, no lice, no worms.....just scraped up backs.
Yes, the feathers did grow back in - it took a LONG while (6wks or more!). What seemed to help the best was to get the game-bird feed with 22% protein. I tried to give them enough supplemental protein (scrambled eggs, yogurt, mealworms), but wasn't ever sure that the ones who needed it the most got the treats! With the gamebird feed, I KNEW they were getting enough protein! In their coop was the regular chick starter feed...in the play run, they had the game-bird feed. That seemed to help the most.
Now, it's hard to tell who had the issues and who didn't. While the lowest of the pecking order is still bald, at least the others are not. And I rehomed the lower on the pecking order scale due to over-crowding.
Hope this helps!