"Customers on our old proxy engine, known as FL, did not see errors, but bot scores were not generated correctly, resulting in all traffic receiving a bot score of zero."

This simply means, the exception handling quality of your new FL2 is non-existent and is not at par / code logic wise similar to FL.

I hope it was not because of AI driven efficiency gains.

In most domains, silently returning 0 in a case where your logic didn't actually calculate the thing you were trying to calculate is far worse than giving a clear error.