Hello there !
I've noticed a bug while playing in prehistorical ages. Quite breaker actualy : Iwas abble to research some technologies without having discovered its prerequesites. The result was a crash when as soon as I had discovered the new technology. In my case I was researching agriculture before stone carving I think?
Actually, I think this MIGHT be the reason why the AI is stagnating in prehistory : maybe the AI choose the most interesting technologies but since they are broken, they are never discovered,. That would explain the fact that the most advanced AI had discovered only two techs at the end of the prehistoric age while producing more science per turn than I did.