Steps to reproduce on regtest:

```
converttopsbt 02000000000101ab73dba46508bacdb49a988ddd74bc9d39c9cb7e057af060ca8c6b6861e28e3c0300000000feffffff02b828d0b20000000017a914d1954862afd728fbb055149002958e1d1b9e1f0b8700ca9a3b0000000017a91442170ccb3441cff1723af9afab6f2553103e1d578702473044022077380c6e16496d9905aef120694c329be51522847cb5d53fa74b94698c95678d02202d32c46e0e17420a83dee830fefda7d40529182f5d82be47c2b49a7b83214ab50121026d483d6628dcc3ef6e3ab9ed4937c452c263c7e0ac80eb4be97426d5da8f640500000000
```
or 
```
converttopsbt 02000000000101ab73dba46508bacdb49a988ddd74bc9d39c9cb7e057af060ca8c6b6861e28e3c0300000000feffffff02b828d0b20000000017a914d1954862afd728fbb055149002958e1d1b9e1f0b8700ca9a3b0000000017a91442170ccb3441cff1723af9afab6f2553103e1d578702473044022077380c6e16496d9905aef120694c329be51522847cb5d53fa74b94698c95678d02202d32c46e0e17420a83dee830fefda7d40529182f5d82be47c2b49a7b83214ab50121026d483d6628dcc3ef6e3ab9ed4937c452c263c7e0ac80eb4be97426d5da8f640500000000 false
```

Further reading: #13738
unknowns.pushKV(HexStr(entry.first), HexStr(entry.second));
            }
            out.pushKV("unknown", unknowns);
        }
        outputs.push_back(out);
        
        output_value += psbtx.tx->vout[i].nValue;
    }
    result.pushKV("outputs", outputs);
    if (have_all_utxos) {
        result.pushKV("fee", ValueFromAmount(total_in - output_value));
    }
    return result;
}
UniValue combinepsbt(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() != 1)
        throw std::runtime_error(
            RPCHelpMan{"combinepsbt",
                "\nCombine multiple partially signed Bitcoin transactions into one transaction.\n"
                "Implements the Combiner role.\n",
                {
                    {"txs", RPCArg::Type::ARR, RPCArg::Optional::NO, "A json array of base64 strings of partially signed transactions",
                        {
                            {"psbt", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "A base64 string of a PSBT"},
                        },
                        },
                },
                RPCResult{
            "  \"psbt\"          (string) The base64-encoded partially signed transaction\n"