I think I would pull that rod and piston out and check the big end for roundness instead of wondering why that bearing failed
Was it checked before assembling the motor? If so what was the bearing to crank clearances?
Any chance of debris in the oil passage to that rod bearing being stuck up in the oil passage in the crankshaft I'm sure the crank journal should polish up well enough to be usable
When I was a lot younger, and a lot less knowledgeable than now, I would have tried to polish that journal in the motor in the car
Now I would remove the crank and take it to a good crankshaft grinding company and have them look at it to see if it would polish and still be useable