That's why it's not usually called "Deutsch self-consistency." It's not supposed to be a filter on legal universes, but a dynamic rule that each initial condition does lead to a consistent universe. The resolution of the grandfather paradox is a 50-50 superposition of the universe where you are born and leave and the universe where you appear, kill your grandfather, and are never born. You could say that it filters out the 80-20 superposition, but that's like saying that Newton's self-consistency principle filters out universes that don't obey his laws. (Well, maybe that's Lagrange's self-consistency principle...)