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.
}, true
    );
    bool rbf = false;
    if (!request.params[3].isNull()) {
        rbf = request.params[3].isTrue();
    }
    CMutableTransaction rawTx = ConstructTransaction(request.params[0], request.params[1], request.params[2], rbf);
    return EncodeHexTx(CTransaction(rawTx));
}
static UniValue decoderawtransaction(const JSONRPCRequest& request)
{
    RPCHelpMan{"decoderawtransaction",
                "\nReturn a JSON object representing the serialized, hex-encoded transaction.\n",
                {
                    {"hexstring", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction hex string"},
                    {"iswitness", RPCArg::Type::BOOL,  "depends on heuristic tests", "Whether the transaction hex is a serialized witness transaction.\n"
                        "If iswitness is not present, heuristic tests will be used in decoding.\n"
                        "If true, only witness deserialization will be tried.\n"
                        "If false, only non-witness deserialization will be tried.\n"
                        "This boolean should reflect whether the transaction has inputs\n"
                        "(e.g. fully valid, or on-chain transactions), if known by the caller."
                    },
                },
                RPCResult{
            "{\n"
            "  \"txid\" : \"id\",        (string) The transaction id\n"
            "  \"hash\" : \"id\",        (string) The transaction hash (differs from txid for witness transactions)\n"
            "  \"size\" : n,             (numeric) The transaction size\n"
            "  \"vsize\" : n,            (numeric) The virtual transaction size (differs from size for witness transactions)\n"
            "  \"weight\" : n,           (numeric) The transaction's weight (between vsize*4 - 3 and vsize*4)\n"