How long were they hung for? We have hung from 2 days up to a week and been fine. Again we’ve dropped off at abattoir the night before and never had a problem, in fact it gives them time to settle from transport which can be stressful in itself.
Technically if the 11 month olds didn’t have their first 2 teeth pushed up they would have been killed as lamb. Once 2 teeth pushed up they have to split the carcass, but it would be hogget and not mutton.
I still reckon it’s an issue with hanging or chilling, or the cooking. Have you said anything to the butcher/abattoir? I guess you paid a fair amount of money for what you say is inedible meat; surely the butcher must have known it wasn’t going to be any good.