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 
TransactionError CWalletFillPSBT(PartiallySignedTransaction& psbtx, bool& complete, int sighash_type, bool sign, bool bip32derivs, size_t * n_signed, bool finalize) const
{
    if (n_signed) {
        *n_signed = 0;
    }
    LOCK(cs_);
    // Get all of the previous transactions
    for (unsigned int i = 0; i < psbtx.tx->vin.size(); ++i) {
        const CTxIn& txin = psbtx.tx->vin[i];
        PSBTInput& input = psbtx.inputs.at(i);
        if (PSBTInputSigned(input)) {
            continue;
        }
        //  we have no utxo, grab it from th.
        if (!input.non_witness_utxo) {
            const uint256& txhash = txin.prevout.hash;
            const auto it = mapWallet.find(txhash);
            if (it != mapWallet.end()) {
                const CWalletTx& wtx = it->second;
                // We only 
                //   The s
                input.non_witness_utxo = wtx.tx;
            }
        }
    }
    const PrecomputedTransactionData txdata = PrecomputePSBTData(psbtx);
    // Fill in information from ScriptPubKeyMans
    for (ScriptPubKeyMan* spk_man  GetAllScriptPubKeyMans()) {
        int n_signed_this_spkm = 0;
        TransactionError res = spk_man->FillPSBT(psbtx, txdata, sighash_type, sign, bip32derivs, &n_signed_this_spkm, finalize);