The babbit metal is supposed to be tinned onto the steel Whether it was or not the end result is caused by fatigue. The thicker the babbit layer the worse it is at taking load. White metal (another name for it) is very forgiving of a bit of muck and should have very small running clearances so it doesn't hammer out and it is low friction so doesn't need high oil flow to cool it . It was almost universal till the WW2 but on "slipper" bearings not straight in the conrod common just before the War..
The modern metals, Alutin and lead bronze (copper lead) need more clearance for cooling oil flow and don't like grit etc and should run on Hardened surfaces or some pretty dreadful things happen . The engines are generally a bit noisier due to the extra clearances.
Bill when I was a kid I had a BSA Z A7 and the RHS bearing didn't last long the way I rode it. I worked at an engine reconditioning place and I removed the bearing carefully and remetalled it about 3 times, over a period, and kept the fit in the case OK. I know a bit more now and would expand the steel bush if it got too loose, or just make a new one. If you flashed the steel bush with nickel (dull, not bright bath) it would tin better and last longer. If you make a new bush reduce the Babbit thicknes to 1/2 mm or a bit less if everything is on centre and the bearing will last longer as it doesn't fatigue as much when thin. The Chevrolet big ends had the whitemetal spun in molten and beautifully tinned. The layer was so thin you couldn't replicate it or you would inevitably strike the steel when on the conrod borer so the steel had to be bored out a bit more to give the latitude for twist etc The repair was never as good as the original in this instance. Nev