My understanding.

When your I485 case was processed, the officer looked back on your approved I140 case and found your I140 was approved prematurely. Therefore an RFE was issued. Since your I140 was approved, the officer can't revoke your approved case. Therefore, even if you ignore the RFE, your I140 case is still approved.
However, the risk of ignoring I140 RFE is that it may cause your I1485 case denied because the officer has the authority to approve or deny your I485. To be safe, consult a lawyer and prepare materials for I140 RFE.

If my memory serves me right, a post about such a thing should be in this forum some time ago.