I sent them all these pictures I took. I said this problem is not seen on the right pieces, nor old version W4Rs. They either omitted this or chose not to answer.
I'm going to send them another email...
Golila3 is right, every W4R has a tiny bump there. But in our case, the stress is so concentrated that the bump becomes a crack (in my opinion).
Yeah, I just checked carefully on my 4r and my "crack" is really small compared to the one on the picture.