Lead doesn’t bounce. It deforms and splits allowing the pretty green kryptonite to pierce Supermans previously invulnerable skin slicing a wound channel and spreading the poison throught out his body.
Anyone who has ever seen Superman have lead bullets shot at him sees the bullets bounce off his chest. And of course bullets ricochet. So, yeah, at the key mokent of impact, the bullet would bounce, and by the time the k-rock was exposed, the payload would be far enough away.