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.
}
        if (!input.missing_redeem_script.IsNull()) {
            missing.pushKV("redeemscript", HexStr(input.missing_redeem_script));
        }
        if (!input.missing_witness_script.IsNull()) {
            missing.pushKV("witnessscript", HexStr(input.missing_witness_script));
        }
        if (!input.missing_sigs.empty()) {
            UniValue missing_sigs_univ(UniValue::VARR);
            for (const CKeyID& pubkey : input.missing_sigs) {
                missing_sigs_univ.push_back(HexStr(pubkey));
            }
            missing.pushKV("signatures", missing_sigs_univ);
        }
        if (!missing.getKeys().empty()) {
            input_univ.pushKV("missing", missing);
        }
        inputs_result.push_back(input_univ);
    }
    result.pushKV("inputs", inputs_result);
    if (psbta.estimated_vsize != nullopt) {
        result.pushKV("estimated_vsize", (int)*psbta.estimated_vsize);
    }
    if (psbta.estimated_feerate != nullopt) {
        result.pushKV("estimated_feerate", ValueFromAmount(psbta.estimated_feerate->GetFeePerK()));
    }
    if (psbta.fee != nullopt) {
        result.pushKV("fee", ValueFromAmount(*psbta.fee));
    }
    result.pushKV("next", PSBTRoleName(psbta.next));