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{"joinpsbts",
            "\nJoins multiple distinct PSBTs with different inputs and outputs into one PSBT with inputs and outputs from all of the PSBTs\n"
            "No input in any of the PSBTs can be in more than one of the PSBTs.\n",
            {
                {"txs", RPCArg::Type::ARR, RPCArg::Optional::NO, "A json array of base64 strings of partially signed transactions",
                    {
                        {"psbt", RPCArg::Type::STR, RPCArg::Optional::NO, "A base64 string of a PSBT"}
                    }}
            },
            RPCResult {
                "  \"psbt\"          (string) The base64-encoded partially signed transaction\n"
            },
            RPCExamples {
                HelpExampleCli("joinpsbts", "\"psbt\"")
            }}.Check(request);
    RPCTypeCheck(request.params, {UniValue::VARR}, true);
    
    std::vector<PartiallySignedTransaction> psbtxs;
    UniValue txs = request.params[0].get_array();
    if (txs.size() <= 1) {
        throw JSONRPCError(RPC_INVALID_PARAMETER, "At least two PSBTs are required to join PSBTs.");
    }
    int32_t best_version = 1;
    uint32_t best_locktime = 0xffffffff;
    for (unsigned int i = 0; i < txs.size(); ++i) {
        PartiallySignedTransaction psbtx;
        std::string error;
        if (!DecodeBase64PSBT(psbtx, txs[i].get_str(), error)) {
            throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed %s", error));