How do I properly define an example for a nullable property in OpenAPI 3?

I am using OpenAPI and openapi-generator-maven-plugin with Spring Boot. I'm trying to create an example object in my response. I've followed the suggestions of several different web pages, especially this one:

https://swagger.io/docs/specification/adding-examples/

But no matter what I try, I can't get the example data to show up correctly. Here the object that should be in a request:

Period:
  type: object
  required:
    - startDate
    - endDate
  properties:
    startDate:
      type: string
      format: date
      nullable: true
      example: "2021-01-01"
    endDate:
      type: string
      format: date
      nullable: true
      example: "2030-01-01"

CreateInstancesRequest:
  type: object
  required:
    - id
  properties:
    id:
      $ref: '#/components/schemas/Id'
    period:
      allOf:
        - $ref: '#/components/schemas/Period'

But I get not what I want:

I expect something like this:

{
  "id": "some-id",
  "period": {
    "endDate": "2020-01-01",
    "startDate": "2022-01-01"
  }
}

But maybe I am wrong because generated java code for the Period class: 

I think present property from actual example came from org.openapitools.jackson.nullable.JsonNullable class.

Can you help me, please?