Since they do test for that, then it seems more likely that this will turn out to be a bone deformity. If that's the case, then it's really sad that the breeder keeps breeding the birds that produce this. 😟
Well, definitely a red flag that he knew legs were a problem. I'm not real familiar with the NPIP program, but that doesn't seem like something that should be acceptable. I just wouldn't buy from him again.
Bringing in 1-3 day old chicks is usually ok, that is usually what you get from feed...
Good luck. Did you hatch these at home, or were they purchased? If they were purchased I would not buy from there again. If it was a reputable breeder or hatchery, I would have a conversation with them about what you've had happen. A reputable breeder or hatchery would want to know if there...
I agree with @Wyorp Rock , get them tested for MG/MS to rule that in or out. The one picture that shows very swollen hocks could be that. The other could be a leg deformity.
If changing brands of feed is an option for you, I'd consider doing that for a while to see if it makes any difference...