swagger-codegen does not generate models for nested objects

We use swagger-codegen v2.3.0 to generate an API client in Python from a yml API spec.
In the case of nested objects, e.g.

definitions:
  Toplevel:
    type: object
    properties:
      id:
        type: string
      Nested:
        type: object
        properties:
          name:
            type: string

It would generate model classes Toplevel and ToplevelNested.

After upgrading to v2.4.18 it does not generate a model for the Nested, and the type of the nested parameter to the Toplevel is declared as just object.

Is it perhaps a change in the default behavior that can be controlled by invocation parameters? Thanks.