pls see below reply from bearing factory
As our engineer’s conclusion, the possibility is as following:
There is a crack on the surface of the inner ring, but it’s Invisible naked eyes.
This crack maybe caused by defeat of material or maybe happened during heat treatment.
The crack should be detected by magnetic detector or supersonic detector, but unfortunately not.
So after installed into the application, there is heave loading on the inner ring, the crack extend then the bearing destroyed