Ask how the pup gets in and out of cars/trucks. If you damage that growth plate, it will stop growing on one side. eventually you will see twisting at the pastern toward that side as the other side keeps growing. Jumping out of cars will do it.

I agree with this. If a puppy is allowed to traumatize a joint (usually elbow or carpus--wrist from jumping out of a vehicle) the growth plate will be permanently altered. Since the puppy did not live with you, and most inexperienced people don't realize the damage they can do with allowing their puppies to jump off of/out of things, that is the most likely cause.