openapi-generator-maven-plugin creates JsonNullable<Object> for properties with ref: #

I've created my .yml (reduced):

openapi: 3.0.1
...
components:
  messages:
  ...
  schemas:
    device:
      type: object
      properties:
        identifier:
          type: string
        manufacturer:
          type: string
        model:
          type: string
    controllerId:
      type: number
    controller:
      type: object
      properties:
        controllerId:
          ref: '#/components/schemas/controllerId'
        device:
          ref: '#/components/schemas/device'

I let generate java classes for Vertx by

<groupId>org.openapitools</groupId>
<artifactId>openapi-generator-maven-plugin</artifactId>
<version>5.3.0</version>

Properties with type: are generated as expected but all properties with ref: are generated as JsonNullable<Object>. Because of that the jackson-ObjectMapper reads the values as LinkedHashMap instead of e. g. Device in Controller's field.

I copied the generated classes and fixed the generics so the mapper reads the values to the expected classes. Do you have any idea how to write the .yml that the generator creates JsonNullable<Device> instead of JsonNullable<Object>? Or is there any configuration of the generator I didn't found yet to solve this problem?