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.
RPCResult {
                "  \"psbt\"          (string) The base64-encoded partially signed transaction with inputs updated\n"
            },
            RPCExamples {
                HelpExampleCli("utxoupdatepsbt", "\"psbt\"")
            }}.Check(request);
    RPCTypeCheck(request.params, {UniValue::VSTR, UniValue::VARR}, true);
    
    PartiallySignedTransaction psbtx;
    std::string error;
    if (!DecodeBase64PSBT(psbtx, request.params[0].get_str(), error)) {
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, strprintf("TX decode failed %s", error));
    }
    
    FlatSigningProvider provider;
    if (!request.params[1].isNull()) {
        auto descs = request.params[1].get_array();
        for (size_t i = 0; i < descs.size(); ++i) {
            EvalDescriptorStringOrObject(descs[i], provider);
        }
    }
    
    HidingSigningProvider public_provider(&provider,  true,  false);
    
    CCoinsView viewDummy;
    CCoinsViewCache view(&viewDummy);
    {
        LOCK2(cs_main, mempool.cs);
        CCoinsViewCache &viewChain = *pcoinsTip;
        CCoinsViewMemPool viewMempool(&viewChain, mempool);