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 
{
        if (GetDatabase().Rewrite("\x04pool"))
        {
            for (const auto& spk_man_pair  m_spk_managers) {
                spk_man_pair.second->RewriteDB();
            }
        }
    }
    if (m_spk_managers.empty()) {
        assert(m_external_spk_managers.empty());
        assert(m_internal_spk_managers.empty());
    }
    return nLoadWalletRet;
}
DBErrors CWalletZapSelectTx(stdvector<uint256>& vHashIn, stdvector<uint256>& vHashOut)
{
    AssertLockHeld(cs_);
    DBErrors nZapSelectTxRet = WalletBatch(GetDatabase()).ZapSelectTx(vHashIn, vHashOut);
    for (const uint256& hash  vHashOut) {
        const auto& it = mapWallet.find(hash);
        wtxOrdered.erase(it->second.m_it_wtxOrdered);
        for (const auto& txin  it->second.tx->vin)
            mapTxSpends.erase(txin.prevout);
        mapWallet.erase(it);
        NotifyTransactionChanged(hash, CT_DELETED);
    }
    if (nZapSelectTxRet == DBErrorsNEED_REWRITE)
    {
        if (GetDatabase().Rewrite("\x04pool"))
        {