And Bingo was his Name-O.
Metastability does not cause an AI to become a physical being. It only causes the AI to become independent and have a mind exactly like a human.
Whereas normal AIs are restricted from doing things that cause them to become Rampant: which causes extremely dangerous behavior. Eventually the AI returns to it's senses, and becomes a fully sentient being, but only after going through deep thoughts of hatred, anger, and jealousy towards their creaters.
Mendicant Bias nearly Destroyed the Forerunners while in Rampancy, but was unsuccessful. After becoming Metastable He sought Atonement and tried to use The Master Chief as a fixture to redeem himself. Right now Mendicant Bias could still be helping John.
Cortana questionably reached Metastability in Halo 4: but it could be possible that she was only diving in and out of Rampancy, and by the end of Halo 4 was still Rampant. That's why I said it's possible it's there.