The only remaining question would be in cases where the understatement was due to a FC that accrues over time (not a PFC.) That might be something like omitted or understated MI renewal premiums.
The official guidance says that in these cases, the reimbursement may be prorated on a straight-line basis (no time value) over the life of the loan and refunded under the lump sum/payment reduction method. I've forgotten any guidance that's specific to loans prepaid by borrowers, but nothing in the Q&A I cited above limits this methodology to active loans.
Using the LS/PR method, the reimbursement is split into two stages. The first stage is the time that passes from consummation until the payment due date immediately following the date you make a partial lump sum payment. From then until the loan terminates, payments are reduced.
If the borrower terminates the loan prematurely (the case at hand), then the payment reductions end prematurely. If the OP's FC error is the "accruing" type, then the LS/PR method will result in a smaller overall reimbursement--because the pro-rated payment reductions will end on payment due date following prepayment of the loan. There can only be a single reimbursement payment to the borrower (because there are no remaining payments due), but the methodology is lump sum/payment reduction, not the "lump sum method."