It might be that they are just highly Hard feathered. Which is what the breed is in Indonesia. Some breeders over there even say their birds are almost bald until a year old, and feathers can be fragile when it’s a hard feathering. The harder feathering is actually a good sign fibro genes are really good. You might hop on Facebook and talk to a few Indonesian breeders. You might be surprised what you find.... Here in the US a lot of the breeders that imported them crossed them early on to a different breed for egg production and it changed the feathering to be softer. So most birds you see here in US are soft feathered and not the appropriate hard feathering you see in full Indonesian birds. I try to stick to only the purer Indonesian lines myself... try to stay away from the crosses the hatcheries did early on. Best wishes...I have not, but they eat the same food as my other birds, (20%starter/grower + grit and occasional mealworms), and NONE of my other breeds have this issue. They have had feather issues since I got them (one of them had NO feathers on his back and they took a while to grow in. They weren't plucked out, they were simply not there, like they hadn't formed.) I will get some good pics later tonight to show you.
Is there a particular food or food item or nutrient that is good for making strong feathers besides protein and calcium? (They do not currently get supplemental calcium - I only give that to my layers.)