The Reg Z, 1026.19, discusses corrective action in two instances after consummation – a “clerical error” or an “event occurring after consummation”. From what you’ve described, finding that the APR is inaccurate after the loan has closed would not fit into either of these.
You would look to the Truth in Lending Act sections 1607 and 1640 for cures. In addition, linked here is an interagency joint statement on restitution you may find helpful: https://www.fdic.gov/regulations/laws/rules/5000-300.html. It is a good idea to involve the bank’s attorney when working through the corrective action.