### 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_
if (solving_data.exists("pubkeys")) {
            for (const UniValue& pk_univ : solving_data["pubkeys"].get_array().getValues()) {
                const std::string& pk_str = pk_univ.get_str();
                if (!IsHex(pk_str)) {
                    throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, strprintf("'%s' is not hex", pk_str));
                }
                const std::vector<unsigned char> data(ParseHex(pk_str));
                const CPubKey pubkey(data.begin(), data.end());
                if (!pubkey.IsFullyValid()) {
                    throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, strprintf("'%s' is not a valid public key", pk_str));
                }
                coinControl.m_external_provider.pubkeys.emplace(pubkey.GetID(), pubkey);
                
                const CScript wit_script = GetScriptForDestination(WitnessV0KeyHash(pubkey));
                coinControl.m_external_provider.scripts.emplace(CScriptID(wit_script), wit_script);
            }
        }
        if (solving_data.exists("scripts")) {
            for (const UniValue& script_univ : solving_data["scripts"].get_array().getValues()) {
                const std::string& script_str = script_univ.get_str();
                if (!IsHex(script_str)) {
                    throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, strprintf("'%s' is not hex", script_str));
                }
                std::vector<unsigned char> script_data(ParseHex(script_str));
                const CScript script(script_data.begin(), script_data.end());
                coinControl.m_external_provider.scripts.emplace(CScriptID(script), script);
            }
        }
        if (solving_data.exists("descriptors")) {
            for (const UniValue& desc_univ : solving_data["descriptors"].get_array().getValues()) {