I was able to reproduce your failure (using Python 3.10) after a couple of runs:

  % grep -v '^aaaaaaaaaa$' kerneloops.out | grep -v '^bbbbbbbbbb$' | od -x
  0000000      8130    0db3    0001    0000    bd30    0dad    0001    0000
  *
  0006420      000a
  0006421

I was surprised as I expected possible interleaved bytes but not output other than "a"s, "b"s and new-lines, as I thought the GIL would come to the rescue.