oas3 kotlin codegenerator: using multipart/form-data and free-form JSON object in the same request

Question

I'm using OAS3 codegenerator to create Kotlin server code for a request containing both a file-upload, and a free-form json object that can contain arbitrary values. According to the swagger documentation, I should be able to create a free-form object that using type: object and additionalProperties : true.

However if I edit the upload photos endpoint in modules/openapi-generator/src/test/resources/3_0/petstore.yaml in the oas3 codegen samples like this

      requestBody:
        content:
          multipart/form-data:
            schema:
              properties:
                file:
                  description: file to upload
                  type: string
                  format: binary
                additionalMetadata:
                  description: Additional data to pass to server
                  type: object                     <-- CHANGED FROM STRING
                  additionalProperties: true       <-- ADDED 

and run ./bin/generate-samples.sh ./bin/configs/kotlin-server-jaxrs-spec.yaml the additionalMetadata field is dropped from PetApi

    @POST
    @Consumes("multipart/form-data")
    @Produces("application/json")
    suspend fun uploadFile(@PathParam("petId") petId: kotlin.Long, @FormParam(value = "file") fileInputStream: InputStream?): Response {

Is there a problem with how I'm specifying the request?