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 
{
        AssertLockHeld(cs_wallet);
        assert(m_last_block_processed_height >= 0);
        return m_last_block_processed_height;
    };
    uint256 GetLastBlockHash() const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet)