[BUG] Missing model files when using embedded/splitted spec files

Description

If I split YML file into 2 files, unused models (not used as a request or response body) are not generated. If everything is in a single file it works just fine.
So for an example below, model files created: Parent.java.
Model files NOT created: ChildOne.java and ChildTwo.java.
If we add e.g. ChildTwo entity into a post response, then it is generated, but ChildOne.java isn't.

openapi-generator version

4.2.2

OpenAPI declaration file content or url

Test project which reveals this issue:
https://github.com/zolv/openapi-generation-bug-report-1

Spec files (splitted):

api-split.yml

openapi: 3.0.2
info:
  title: API Documentation
  version: 1.0.0
servers:
  - url: http://localhost:8080/
paths:
  /users:
    $ref: './api-split-sub.yml#/test'

api-split-sub.yml

test:
   post:
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/Parent'
      responses:
        200:
          description: It's OK
    
components:
  schemas:
  
    Parent:
      required:
        - status
      properties:
        status:
          type: string
      discriminator:
        propertyName: status
        mapping:
          CHILD_ONE: '#/components/schemas/ChildOne'
          CHILD_TWO: '#/components/schemas/ChildTwo'

    ChildOne:
      properties:
        status:
          type: string
      discriminator:
        - $ref: '#/components/schemas/Parent'
        - type: object
        
    ChildTwo:
      allOf:
        - $ref: '#/components/schemas/Parent'
        - type: object

Command line used for generation

./gradlew clean openApiGenerate -s

Steps to reproduce

Nothing. It's just a command line execution.

Related issues/PRs

Nothing I could find.
My issue was first asked on SO

Suggest a fix

No suggestion for fix.
WOrkaround: put everything into single file.

Start with the linked test project and its docs/openapi/api-split.yml and api-split-sub.yml files, then run ./gradlew clean openApiGenerate -s to reproduce the generation. Compare the split-spec output with the single-file workaround; the work is done when ChildOne.java and ChildTwo.java are generated from the split specification.