When the newly introduced `"maxfeerate"` option is not passed, the default fee rate is interpreted as an absolute amount. However, when it is passed, it is interpreted as a fee rate. Thus, the rpc will return different results when the value is explicitly set to the default.

See

https://github.com/bitcoin/bitcoin/blob/536590f358dc3d3e5821eba7f1009452ea93b205/src/rpc/rawtransaction.cpp#L797-L807

I think we should change it back to an absolute value in BTC (not a feerate)

This was changed to a feerate by @kallewoof, so ping him to see if there was a specific reason to switch.
CCoinsViewMemPool viewMempool(&viewChain, mempool);
        view.SetBackend(viewMempool); 
        for (const CTxIn& txin : mergedTx.vin) {
            view.AccessCoin(txin.prevout); 
        }
        view.SetBackend(viewDummy); 
    }
    
    
    const CTransaction txConst(mergedTx);
    
    for (unsigned int i = 0; i < mergedTx.vin.size(); i++) {
        CTxIn& txin = mergedTx.vin[i];
        const Coin& coin = view.AccessCoin(txin.prevout);
        if (coin.IsSpent()) {
            throw JSONRPCError(RPC_VERIFY_ERROR, "Input not found or already spent");
        }
        SignatureData sigdata;
        
        for (const CMutableTransaction& txv : txVariants) {
            if (txv.vin.size() > i) {
                sigdata.MergeSignatureData(DataFromTransaction(txv, i, coin.out));
            }
        }
        ProduceSignature(DUMMY_SIGNING_PROVIDER, MutableTransactionSignatureCreator(&mergedTx, i, coin.out.nValue, 1), coin.out.scriptPubKey, sigdata);
        UpdateInput(txin, sigdata);
    }
    return EncodeHexTx(CTransaction(mergedTx));
}
static UniValue signrawtransactionwithkey(const JSONRPCRequest& request)