Your internal auditor may be expressing a personal opinion. Issuing the exemption was voluntary i.e. you did not need any reason to revoke it, i.e. "It was Tuesday and I didn't have anything else to do" would do nicely. So, I'm not certain why you would be criticized for failing to document the "why" of revocation as long as the fact of revocation was properly communicated to affected personnel. It may be that your BSA policy expressly requires issuance of exemptions wherever they are avaialable and, thus, if one is revoked there is an implication that the policy implies a reason is necessary.
The instructions to the current form do not require you to notify the government that it has been revoked, but even if you do send it voluntarily, the form does not ask "why."