The absolute, final deadline for bank to resolve a billing error dispute under the Fair Credit Billing Act is 90 days from date initial dispute letter (OP, you did send a U.S. Mail letter to special billing error address on your bill, didn't you?) is received.

It appears that the deadline is now past and the bank cannot lawfully recharge the amount.

I had the unlawful recharge thing happen years ago with a disputed charge for a car rental in Europe. Correct bill would have been $200, car rental company put through $1200, I disputed charge using FCRA dispute procedure, got "temporary" credit, then a few months later, recharged for entire amount. A call to customer service pointing out the violation resulted in immediate final credit to my account.