[openapi spec 3.1.0][python-flask generator] Nullable properties not propagated to child models

Description

Currently, we use openapi spec 3.0.3 with openapi-generator version 5.4.0 and we are planning to migrate to openapi spec 3.1.0 with the latest generator version.

While migrating to 3.1.0 I see that nullable is not supported anymore and we use the isNullable value to set our generated model instance variable to Optional as follows:

{{#isNullable}}Optional[{{/isNullable}}{{#isDateTime}}Union[datetime, DateTimeProxy]{{/isDateTime}}{{^isDateTime}}{{dataType}}{{/isDateTime}}{{#isNullable}}]{{/isNullable}}

I noticed that with the new version of the generator and openapi spec 3.1.0 when we set a property as "null" this constraint is not inherited by the child schema in the model generated. Here is a simple example:

Openapi spec 3.1.0:

openapi: 3.1.0
paths:
  /users/{userId}:
    get:
      responses:
        '200':
          description: Successful response
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/UserResponse'

components:
  schemas:
    BaseUser:
      type: object
      properties:
        id:
          type:
            - string
            - 'null'
        username:
          type: string

    UserResponse:
      type: object
      allOf:
        - $ref: '#/components/schemas/BaseUser'
        - type: object
          properties:
            email:
              type: string
              description: The email of the user

Model generated:
base_user.py

class BaseUser(TypedDict):
    id: Optional[str]
    username: str

user_response.py

class UserResponse(TypedDict):
    id: str
    username: str
    email: str

Notice how the id : Optional[str] is not the same as parent in the child schema.
This worked as expected with 3.0.3 and the new version of the generator as well. So makes me wonder if the 3.1.0 spec isNullable property is propagated to the child schema.

Please let me know if there is a way I can resolve this as we want to utilize the great features in 3.1.0 without breaking our client code.

openapi-generator version

7.8.0

OpenAPI declaration file content or url

Command line used for generation

openapi-generator-cli generate --input-spec spec/bundle.yaml --config openapi-generator.yaml --output ../..

Note: the openapi-generator.yaml config file is straightforward and no options are enabled in it.

Start with the supplied OpenAPI 3.1.0 schema and the openapi-generator-cli command using the python-flask generator. Compare the generated base_user.py and user_response.py models, focusing on the nullable id property across allOf inheritance. Done means the child model preserves the parent property's Optional typing without breaking the existing 3.0.3 behavior.