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 
// I
        if (coin_control && coin_control->HasInputWeight(txin.prevout)) {
            if (!FillInputToWeight(txin, coin_control->GetInputWeight(txin.prevout))) {
                return false;
            }
            nIn++;
            continue;
        }
        const stdunique_ptr<SigningProvider> provider = GetSolvingProvider(txout.scriptPubKey);
        if (!provider || !DummySignInput(*provider, txin, txout, coin_control)) {
            if (!coin_control || !DummySignInput(coin_control->m_external_provider, txin, txout, coin_control)) {
                return false;
            }
        }
        nIn++;
    }
    return true;
}
bool CWalletImportScripts(const stdset<CScript> scripts, int64_t timestamp)
{
    auto spk_man = GetLegacyScriptPubKeyMan();
    if (!spk_man) {
        return false;
    }
    LOCK(spk_man->cs_KeyStore);
    return spk_man->ImportScripts(scripts, timestamp);
}
bool CWalletImportPrivKeys(const stdmap<CKeyID, CKey>& privkey_map, const int64_t timestamp)
{
    auto spk_man = GetLegacyScriptPubKeyMan();