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 
}
// Helper for producing a max-sized low-S low-R signature (eg 71 bytes)
// or a max-sized low-S signature (e.g. 72 bytes) depending on coin_control
bool DummySignInput(const SigningProvider& provider, CTxIn &tx_in, const CTxOut &txout, const CCoinControl* coin_control)
{
    // F
    const CScript& scriptPubKey = txout.scriptPubKey;
    SignatureData sigdata;
    // U
    // t
    const bool use_max_sig = coin_control && (coin_control->fAllowWatchOnly || coin_control->IsExternalSelected(tx_in.prevout));
    if (!ProduceSignature(provider, use_max_sig ? DUMMY_MAXIMUM_SIGNATURE_CREATOR  DUMMY_SIGNATURE_CREATOR, scriptPubKey, sigdata)) {
        return false;
    }
    UpdateInput(tx_in, sigdata);
    return true;
}
bool FillInputToWeight(CTxIn& txin, int64_t target_weight)
{
    assert(txin.scriptSig.empty());
    assert(txin.scriptWitness.IsNull());
    int64_t txin_weight = GetTransactionInputWeight(txin);
    // D
    if (target_weight < txin_weight) {
        return false;
    }
    if (target_weight == txin_weight) {
        return true;
    }
    // S