Kong Gateway - Updating Request Json Using Request-Transformer Plugin

I am using Kong 3.3.0 in db-less mode using declarative yaml.

I'm trying to update the request body with a value present in the headers.

If I set a top-level JSON attribute it all works OK. When I try and update the value in a sub-object within the JSON then it fails; well, it sets the value but not exactly where I need it to be).

I am not sure if I am just specifying the JSON path incorrectly in the config or if this plugin doesn't actually support this functionality.

The relevant part of my kong.yaml when it works looks as follows:

  - host: spring-template
    name: spring_template_service
    port: 12345
    protocol: http
    routes:
      - name: spring_example
        paths:
          - /spring/example
        strip_path: false
        plugins:
          - name: request-transformer
            enabled: yes
            config:
              add:
                body:
                  - newkey:newvalue
              replace:
                body:
                  - newkey:newvalue

When I pass in a request payload that looks as follows then the "newkey" attribute is either added if not present, or updated it it is present.

So for an empty input of:

{
}

it correctly returns:

{
   "newkey": "newvalue"
}

I have confirmed this by inspecting the raw JSON that arrives at my Spring Boot service that Kong forwards the request onto.

As I say, I'm not sure how/if you can specify a sub object in the JSON but if I modify the relevant parts of the config above to be:

            config:
              add:
                body:
                  - newkey.subkey:newvalue
              replace:
                body:
                  - newkey.subkey:newvalue

then in the JSON that is output from Kong I see this:

{
   "newkey.subkey": "newvalue"
}

whereas what I was hoping for was:

{
   "newkey": {
      "subkey": "newvalue"
   }
}

Does anyone know if the request-transformer plugin from Kong support this? The Kong documentation is pretty sparse and only covers very simple use-cases.

I have tried different patterns of config but none of them seem to give me what I need.

The only other option open is to write my own plugin to do what I need.