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.
"]\n"
                },
                RPCExamples{
            "\nCreate a transaction\n"
            + HelpExampleCli("createrawtransaction", "\"[{\\\"txid\\\" : \\\"mytxid\\\",\\\"vout\\\":0}]\" \"{\\\"myaddress\\\":0.01}\"") +
            "Sign the transaction, and get back the hex\n"
            + HelpExampleCli("signrawtransactionwithwallet", "\"myhex\"") +
            "\nTest acceptance of the transaction (signed hex)\n"
            + HelpExampleCli("testmempoolaccept", "[\"signedhex\"]") +
            "\nAs a JSON-RPC call\n"
            + HelpExampleRpc("testmempoolaccept", "[\"signedhex\"]")
                },
    }.Check(request);
    RPCTypeCheck(request.params, {
        UniValue::VARR,
        UniValueType(), 
    });
    if (request.params[0].get_array().size() != 1) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Array must contain exactly one raw transaction for now");
    }
    CMutableTransaction mtx;
    if (!DecodeHexTx(mtx, request.params[0].get_array()[0].get_str())) {
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "TX decode failed");
    }
    CTransactionRef tx(MakeTransactionRef(std::move(mtx)));
    const uint256& tx_hash = tx->GetHash();
    CFeeRate max_raw_tx_fee_rate = DEFAULT_MAX_RAW_TX_FEE_RATE;
    
    if (request.params[1].isBool()) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "Second argument must be numeric (maxfeerate) and no longer supports a boolean. To allow a transaction with high fees, set maxfeerate to 0.");