Extending a schema using discriminator does not work

Schemas can be extended with new parameters using allOf, however, if you try to extend a schema which uses discriminator, the new parameters are not shown.

In the example below, if I extend #/components/schemas/Simple ReDoc works as expected, but if I extend #/components/schemas/Pet then the extension field is not present in the resulting doc.

Workaround is to duplicate the discriminated schema as many times as you need (in my case three), which can mean quite a lot of duplicated lines.

openapi: 3.0.0
info:
  title: Test
  version: 1.0.0

components:
  schemas:
    Simple:
      type: object
      properties:
        simple:
          type: string
    Pet:
      type: object
      required:
      - pet_type
      properties:
        pet_type:
          type: string
      discriminator:
        propertyName: pet_type
    Cat:
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        # all other properties specific to a `Cat`
        properties:
          name:
            type: string
    Dog:
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        # all other properties specific to a `Dog`
        properties:
          bark:
            type: string

paths:
  /rest/stuff:
    post:
      summary: Create extended pet
      requestBody:
        content:
          application/json:
            schema:
              allOf:
                # If referencing /Simple the extension works, but not on /Pet:
                - $ref: '#/components/schemas/Pet'
                - type: object
                  properties:
                    extension:
                      type: string

      responses:
        "200":
          description: pet is created

No source file or test is named. Start by reproducing the provided OpenAPI example and trace how ReDoc resolves and renders allOf schemas with a discriminator. Done means the extension field appears when Pet is extended without duplicating the discriminated schema.