I've seen guys forget to polish the crank and just as important the oil holes will need to be de burred chews a lot of debris out in a hurry and bearings go bad form there.

then
I've seen guys nick the crank on the main studs as they drop it in? buddy did it once and the bearings started looked to like that after a short break in he found the debris in his oil filter check then pulled the pan had to pull his motor back out.

Around the same time A guy that works in my garage behind the house did the same thing! I found it after turning motor over a bit by hand and found my aluminum rods although clearing had next to 0 clearance when I pulled the rods low and behold the nicks where pulling debris off the bearings already and causing quite a mess and i had just been turning it by hand.
Anyways little file to take nicks off re polish crank new bearings and all was fine glad I found it before i ran it!
Next time I put plastic covers over main studs (crank girdle they are real long!)


Anyway I would have kicked his butt but hard to kick your own butt!

I'll bet it was one of the above and or the block prep.

Check and re polish the crank clean the block new bearings and I'll bet your just fine!