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 
for (const CTxIn& txin  wtx.tx->vin)
    {
        if (mapTxSpends.count(txin.prevout) <= 1)
            continue;  // N
        range = mapTxSpends.equal_range(txin.prevout);
        for (TxSpendsconst_iterator _it = range.first; _it != range.second; ++_it)
            result.insert(_it->second);
    }
    return result;
}
bool CWalletHasWalletSpend(const CTransactionRef& tx) const
{
    AssertLockHeld(cs_);
    const uint256& txid = tx->GetHash();
    for (unsigned int i = 0; i < tx->vout.size(); ++i) {
        if (IsS(COutPoint(txid, i))) {
            return true;
        }
    }
    return false;
}
void CWalletFlush()
{
    GetDatabase().Flush();
}
void CWalletClose()
{
    GetDatabase().Close();
}
void CWalletSyncMetaData(stdpair<TxSpendsiterator, TxSpendsiterator> range)