On iOS mobile dynamic shadows seems to use wrap mode repeat in sampler. Shadow sampler should never use this addressing mode for obvious reasons(dublicate shadows on wrong places). Because Metal is missing Clamp to Border color maybe clamp_to_edge with 1 pixel scissors borders should be used.
Edit: https://developer.apple.com/library/ios/documentation/Metal/Reference/MetalShadingLanguageGuide/data-types/data-types.html#//apple_ref/doc/uid/TP40014364-CH2-SW13