I explained what the latest state of that bug is over here: https://itch.io/t/150983/somthing-seems-off-in-the-pick-best-creature-algorithm-...
Effectively, the bug isn't that the best creature isn't being selected correctly, but that the best creature sometimes decides to just not repeat what it did really well in the previous generation, even though it has the exact same brain as before. Makes the whole thing much much more complicated to try to fix.