It seems that it would otherwise silently skip the rescan for pre-backup used keys that are still waiting to be generated into the keypool?

See also #11249 
LOCK(cs_);
        for (const MasterKeyMapvalue_type& pMasterKey  mapMasterKeys)
        {
            if(!crypter.SetKeyFromPassphrase(strWalletPassphrase, pMasterKey.second.vchSalt, pMasterKey.second.nDeriveIterations, pMasterKey.second.nDerivationMethod))
                return false;
            if (!crypter.Decrypt(pMasterKey.second.vchCryptedKey, _vMasterKey))
                continue; // t
            if (Unlock(_vMasterKey, accept_no_keys)) {
                // N
                UpgradeKeyMetadata();
                // N
                UpgradeDescriptorCache();
                return true;
            }
        }
    }
    return false;
}
bool CWalletChangeWalletPassphrase(const SecureString& strOldWalletPassphrase, const SecureString& strNewWalletPassphrase)
{
    bool fWasLocked = IsLocked();
    {
        LOCK(cs_);
        Lock();
        CCrypter crypter;
        CKeyingMaterial _vMasterKey;
        for (MasterKeyMapvalue_type& pMasterKey  mapMasterKeys)
        {
            if(!crypter.SetKeyFromPassphrase(strOldWalletPassphrase, pMasterKey.second.vchSalt, pMasterKey.second.nDeriveIterations, pMasterKey.second.nDerivationMethod))
                return false;