Ok this one was fun. Explanation:

We were missing the following GitLab CI rule definition as part of bump_on_airflow_dags:

  # Don't allow bumping from the MR pipeline.
- if: $CI_PIPELINE_SOURCE == 'merge_request_event'
  when: never

For our purposes, there are two different pipeline triggers: push (when we push to a branch) and merge_request_event (when we create or otherwise modify an MR).

When we define the above rule on the bump_on_airflow_dags job, we are implicitly telling Gitlab that there is no need to create a separate pipeline when the MR is created. Thus Gitlab behavior is to bind the pipeline run for the push event with the MR creation. If we do *not* have the above flag, then Gitlab would create separate pipeline runs, but, for some strange reason, never run but 'skip' the one associated with the MR. Go figure. ¯\_(ツ)_/¯

Anyhow, I've fixed this on both image_suggestions and on section_topics.