### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current behaviour

See inline discussion at https://github.com/bitcoin/bitcoin/pull/26467#discussion_r1269177446

@whitslack:

> Consider the case where I am paying someone on chain under the stipulation that _they_ will eat the mining fee. (Thus, I specify _their_ address in `subtractfeefrom` when I'm constructing my transaction.) Later, they tell me that they can't wait for confirmation any longer and need an urgent fee bump. If I naïvely specify their output as the `reduce_output`, I may end up paying them _**much more**_ than I intended to if I only have large UTxOs in my wallet.


### Expected behaviour

Not sure.

Improving the docs is one option, but this behaviour seem quite counter intuitive.

My own suggestion:

> Should we disable the adding of new inputs when `reduce_output` is set? If someone really intends to do that, they should probably use `outputs`.

### Steps to reproduce

Haven't tried.

### Relevant log output

_No response_

### How did you obtain Bitcoin Core

Compiled from source

### What version of Bitcoin Core are you using?

Master after #26467

### Operating system and version

Any

### Machine specifications

_No response_
{
    UniValue params(UniValue::VOBJ);
    UniValue positional_args{UniValue::VARR};
    for (std::string_view s: strParams) {
        size_t pos = s.find('=');
        if (pos == std::string::npos) {
            positional_args.push_back(rpcCvtTable.ArgToUniValue(s, strMethod, positional_args.size()));
            continue;
        }
        std::string name{s.substr(0, pos)};
        std::string_view value{s.substr(pos+1)};
        
        
        
        params.pushKV(name, rpcCvtTable.ArgToUniValue(value, strMethod, name));
    }
    if (!positional_args.empty()) {
        
        
        
        params.pushKVEnd("args", positional_args);
    }
    return params;
}