Conditional rule always trigger job

Version used: gitlab.com (free tier)

I have this rule on a job

  rules:
    - if: '$CI_PIPELINE_SOURCE == "trigger"'
      when: never

The way I understand it (which is also what I want it to do), is that it should prevent the job from being added to the pipeline when the contents of variable $CI_PIPELINE_SOURCE equals trigger.

This is not the case, and the job in question is always removed from the pipeline.

The pipeline is triggered by an API call from another pipeline https://docs.gitlab.com/ee/ci/triggers/

What am I doing wrong ?

Edit

The variable value is displayed on the pipeline screen

I think this is the correct behaviour as per documentation here:

As in this example here you probably need to include the when: on_success

job:
  script: echo "Hello, Rules!"
  rules:
    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
      when: never
    - if: $CI_PIPELINE_SOURCE == "schedule"
      when: never
    - when: on_success