I've noticed that if you define a Schema using allOf, that particular schema will note marked as an object itself, even though it's based in other objects.

OAS3 States:

Composition and Inheritance (Polymorphism)

The OpenAPI Specification allows combining and extending model definitions using the allOf property of JSON Schema, in effect offering model composition. allOf takes an array of object definitions that are validated independently but together compose a single object.

It's clear that the result of a usage of an allOf statement should produce an object.

Tried with:

components:
  schemas:
    Error:
      type: object,
      properties:
        message:
          type: string
    TS:
      type: object
      properties:
        moment:
          type: string,
          format: datetime
    ExtendedError:
      type: object
      properties:
        message:
          type: string
        suberror:
          allOf:
            - $ref: '#/components/schemas/Error'
            - $ref: '#/components/schemas/TS'

In this case the Schema for suberror have it's type property set to SchemaType.Any because of they way how the type is defined without checking on inheritance:

self.type = SchemaType(schema_type)
# ... REDACTED CODE ...
self.all_of = all_of and list(all_of) or []