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{"converttopsbt",
                "\nConverts a network serialized transaction to a PSBT. This should be used only with createrawtransaction and fundrawtransaction\n"
                "createpsbt and walletcreatefundedpsbt should be used for new applications.\n",
                {
                    {"hexstring", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The hex string of a raw transaction"},
                    {"permitsigdata", RPCArg::Type::BOOL,  "false", "If true, any signatures in the input will be discarded and conversion\n"
                            "                              will continue. If false, RPC will fail if any signatures are present."},
                    {"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{
                            "  \"psbt\"        (string)  The resulting raw transaction (base64-encoded string)\n"
                },
                RPCExamples{
                            "\nCreate a transaction\n"
                            + HelpExampleCli("createrawtransaction", "\"[{\\\"txid\\\":\\\"myid\\\",\\\"vout\\\":0}]\" \"[{\\\"data\\\":\\\"00010203\\\"}]\"") +
                            "\nConvert the transaction to a PSBT\n"
                            + HelpExampleCli("converttopsbt", "\"rawtransaction\"")
                },
    }.Check(request);
    RPCTypeCheck(request.params, {UniValue::VSTR, UniValue::VBOOL, UniValue::VBOOL}, true);
    
    CMutableTransaction tx;
    bool permitsigdata = request.params[1].isNull() ? false : request.params[1].get_bool();