Stageless pipeline fails on non-existent stage

GitLab CI/CD
1

Stageless pipeline fails on a job not present in any previous stage

What are we trying to do:
We want to have pipeline templates defined in one templates repository globally for the company. These templates may then be used in a different CI repository to manage all our team’s CI pipelines. Finally, in project repositories, we use these pipelines to deploy (infrastructure as) code.
We want to move from stageful to stageless pipelines. For some time, this might mean mixing stageless and stageful jobs in one CI pipeline like in this case.

The lineage goes like this:

Templates repository

terraform.src.yml  # This file defines the basic hidden jobs without relations
terraform.yml      # Visible jobs that extend the src jobs and add relations

CI repository

sandbox.yml  # This file contains all the CI jobs for the specific project

Project repository

CI/CD configuration file is sourced from CI repository

CI files:
terraform.src.yml

.image: &image
  image: $CI_REGISTRY/platform/runner/terraform:latest

.tf_init: &tf_init
  - cd .
  - terraform init

.cache: &cache
  key:
    files:
      - versions.tf
  paths:
    - ./.terraform/providers/
    - ./.terraform.lock.hcl

###### Hidden jobs (to be extended) ######
.tf_init_cache:
  <<: *image
  cache:
    <<: *cache
  script:
    - *tf_init
  rules:
    - changes:
        - "**/versions.tf"

.tf_plan:
  <<: *image
  cache:
    <<: *cache
    policy: pull
  script:
    - *tf_init
    - terraform plan -out default.tf_plan   
  artifacts:
    paths:
      - default.tf_plan

.tf_apply:
  <<: *image
  cache:
    <<: *cache
    policy: pull
  script:
    - *tf_init
    - terraform apply -auto-approve default.tf_plan
  rules:
    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH

.except_schedules_pipelines:
  rules:
    - if: $CI_PIPELINE_SOURCE == "schedule"
      when: never
    - if: $CI_PIPELINE_SOURCE == "pipeline"
      when: never

terraform.yml
Jobs without relations in the previous file are extended with relations in this one.

include:
  - https://ci-files.example.com/infrastructure/terraform.src.v1.yml

tf_init_cache:
  extends:
    - .tf_init_cache
  needs: []

tf_plan:
  extends:
    - .tf_plan
  needs:
    - tf_init_cache

tf_apply:
  extends:
    - .tf_apply
  needs:
    - tf_plan
  allow_failure: false

sandbox.yml
Now we are actually defining the whole pipeline in CI repository

stages:
  - lint
  - apply
  - k8s-system
  - k8s

include:
  - "https://ci-files.example.com/infrastructure/terraform.v1.yml"

tf plan:
  extends:
    - tf_plan
    - .except_schedules_pipelines

tf apply:
  extends:
    - tf_apply
    - .except_schedules_pipelines
  rules:
      # allow manual apply also from powersuer/* branches. Data team does a lot of testing
    - if: $CI_COMMIT_BRANCH =~ /poweruser\/\S+/
      when: manual

# some other jobs using the above stages

What we expect:
Pipeline runs without problems, in stage view all jobs without stage are executed in test stage.
Dependencies are defined and the jobs in test run according to the defined order

What actually happens:
We see the following error in our project repository:
infra validator job: chosen stage does not exist; available stages are .pre, lint, apply, k8s-system, k8s, .post

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