To borrow some definitions from Systems engineering for verification and validation, this question is one of validation. Verification is performed by Lean and spec syntax and logic enforcement. But Validation is a question of is if the Lean spec encodes a true representation of the problem statement (was the right thing specced). Validation at highest levels is probably an irreplaceable human activity.

Also, on the verification side - there could also be a window of failure that Lean itself has a hidden bug in it too. And with automated systems that seek correctness, it is slightly elevated that some missed crack of a bug becomes exploited in the dev-check-dev loop run by the AI.