I dug further and found this:

If the parent pipeline is a merge request pipeline, the child pipeline must use workflow:rules or rules to ensure the jobs run.

If no jobs in the child pipeline can run due to missing or incorrect rules configuration:

Adding following rules section to the jobs fixes my issue (This is because I need the child pipelines to run on a merge request only):

  rules:
    - if: $CI_MERGE_REQUEST_ID