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 (g_txindex && !pblockindex) {
        g_txindex->BlockUntilSyncedToCurrentChain();
    }
    LOCK(cs_main);
    if (pblockindex == nullptr)
    {
        CTransactionRef tx;
        if (!GetTransaction(oneTxid, tx, Params().GetConsensus(), hashBlock) || hashBlock.IsNull())
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Transaction not yet in block");
        pblockindex = LookupBlockIndex(hashBlock);
        if (!pblockindex) {
            throw JSONRPCError(RPC_INTERNAL_ERROR, "Transaction index corrupt");
        }
    }
    CBlock block;
    if(!ReadBlockFromDisk(block, pblockindex, Params().GetConsensus()))
        throw JSONRPCError(RPC_INTERNAL_ERROR, "Can't read block from disk");
    unsigned int ntxFound = 0;
    for (const auto& tx : block.vtx)
        if (setTxids.count(tx->GetHash()))
            ntxFound++;
    if (ntxFound != setTxids.size())
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Not all transactions found in specified or retrieved block");
    CDataStream ssMB(SER_NETWORK, PROTOCOL_VERSION | SERIALIZE_TRANSACTION_NO_WITNESS);
    CMerkleBlock mb(block, setTxids);
    ssMB << mb;
    std::string strHex = HexStr(ssMB.begin(), ssMB.end());
    return strHex;
}
static UniValue verifytxoutproof(const JSONRPCRequest& request)