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.
{
    RPCHelpMan{"testmempoolaccept",
                "\nReturns result of mempool acceptance tests indicating if raw transaction (serialized, hex-encoded) would be accepted by mempool.\n"
                "\nThis checks if the transaction violates the consensus or policy rules.\n"
                "\nSee sendrawtransaction call.\n",
                {
                    {"rawtxs", RPCArg::Type::ARR, RPCArg::Optional::NO, "An array of hex strings of raw transactions.\n"
            "                                        Length must be one for now.",
                        {
                            {"rawtx", RPCArg::Type::STR_HEX, RPCArg::Optional::OMITTED, ""},
                        },
                        },
                    {"maxfeerate", RPCArg::Type::AMOUNT,  FormatMoney(DEFAULT_MAX_RAW_TX_FEE_RATE.GetFeePerK()), "Reject transactions whose fee rate is higher than the specified value, expressed in " + CURRENCY_UNIT + "/kB\n"},
                },
                RPCResult{
            "[                   (array) The result of the mempool acceptance test for each raw transaction in the input array.\n"
            "                            Length is exactly one for now.\n"
            " {\n"
            "  \"txid\"           (string) The transaction hash in hex\n"
            "  \"allowed\"        (boolean) If the mempool allows this tx to be inserted\n"
            "  \"reject-reason\"  (string) Rejection string (only present when 'allowed' is false)\n"
            " }\n"