Hmmm . . I'll have a guess and say it might be a suboptimal piston/bore fit.
I had a few sleeves pinched by osrocket and they were extremely tight, I thought. So much so that I had lots of "tinking" noise as the piston got caught at TDC. The "tinking" was most apparent when the engine was revving slowly, say, just before it was cutting out. The recently replaced rear bearing was toast.
I figured the stresses were rather more than I was happy with and ended up doing some amateur lapping of the piston with some paste type bath cleaner - took the piston up about .5mm to about 3.5mm. I was at it for ages with that bath cleaner.
I restarted the break in and it proceeded fine from there.