I still say that's a crappy two-shot injection mold. What happened is simply the black plastic didn't fully fill the entire mold as it should have. It happens sometimes. It's a bad part. Maybe their pressure was off when running that batch.
IMO what some are calling a crack is actually two injected plastic flows that never met up and joined together fully. You wouldn't notice anything unusual at first but eventually as you wear away some top layer of the black plastic, you're noticing that the black plastic isn't as solid as it should have been. Where ever you don't have the black plastic, you have the clear plastic, so hence the light is shining through.