book a reverse provision to the ACL.

Lots of reasons this may be the case.

The specific loan performance might have improved, the credit worthiness of the borrower might have improved, or if measured on a portfolio basis, something like an improved economy might lower your expectation of future credit losses for a loan portfolio might necessitate a reverse provision. The original reply is correct – you need to have good support for the improved outlook, but honestly this calculation is so judgmental that you have to have good support for whatever choice you decide, regardless of the direction of the allowance