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.
}
        psbtxs.push_back(psbtx);
        
        if (psbtx.tx->nVersion > best_version) {
            best_version = psbtx.tx->nVersion;
        }
        
        if (psbtx.tx->nLockTime < best_locktime) {
            best_locktime = psbtx.tx->nLockTime;
        }
    }
    
    PartiallySignedTransaction merged_psbt;
    merged_psbt.tx = CMutableTransaction();
    merged_psbt.tx->nVersion = best_version;
    merged_psbt.tx->nLockTime = best_locktime;
    
    for (auto& psbt : psbtxs) {
        for (unsigned int i = 0; i < psbt.tx->vin.size(); ++i) {
            if (!merged_psbt.AddInput(psbt.tx->vin[i], psbt.inputs[i])) {
                throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Input %s:%d exists in multiple PSBTs", psbt.tx->vin[i].prevout.hash.ToString().c_str(), psbt.tx->vin[i].prevout.n));
            }
        }
        for (unsigned int i = 0; i < psbt.tx->vout.size(); ++i) {
            merged_psbt.AddOutput(psbt.tx->vout[i], psbt.outputs[i]);
        }
        merged_psbt.unknown.insert(psbt.unknown.begin(), psbt.unknown.end());
    }
    CDataStream ssTx(SER_NETWORK, PROTOCOL_VERSION);
    ssTx << merged_psbt;