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 
* ReserveDestination is used to reserve an address.
 * It is currently only used inside of CreateTransaction.
 *
 * Instantiating a ReserveDestination does not reserve an address. To do so,
 * GetReservedDestination() needs to be called on the object. Once an address has been
 * reserved, call KeepDestination() on the ReserveDestination object to make sure it is not
 * returned. Call ReturnDestination() to return the address so it can be re-used (for
 * example, if the address was used in a new transaction
 * and that transaction was not completed and needed to be aborted).
 *
 * If an address is reserved and KeepDestination() is not called, then the address will be
 * returned when the ReserveDestination goes out of scope.
 */
class ReserveDestination
{
protected:
    
    const CWallet* const pwallet;
    
    ScriptPubKeyMan* m_spk_man{nullptr};
    OutputType const type;
    
    int64_t nIndex{-1};
    
    CTxDestination address;
    
    bool fInternal{false};
public:
    
    explicit ReserveDestination(CWallet* pwallet, OutputType type)