Child pipeline does not run for web or scheduled pipelines

Summary

When using the parent/child pipeline, the child pipeline does not get run via the web or scheduled trigger.

Steps to reproduce

1. Create the parent gitlab-ci.yml file

   parent:
     trigger:
       include: child/.gitlab-ci.yml
       strategy: depend
     rules:
       - if: '$CI_PIPELINE_SOURCE == "web"'
       - if: '$CI_PIPELINE_SOURCE == "schedule"'

2. Create the child gitlab-ci.yml file

   variables:
     CHILD_PATH: child
   
   before_script:
     - cd $CHILD_PATH
   
   build:
     image: node:12
     only:
       refs:
         - web
     stage: build
     script:
       - touch done.txt

3. Run the pipeline via Run Pipeline web interface

Example Project

https://gitlab.com/primoz.hadalin/gitlab-parent-child-pipeline-fail

Without the "only" part in the child yml file

only:
  refs:
    - web

the pipeline runs successfully (see pipeline #256290683)

But with the "only" rule the pipeline doesn't start (see pipeline #256293033) due to "No stages / jobs for this pipeline" error.

What is the current bug behavior?

The child pipeline is not run.

What is the expected correct behavior?

The child pipeline should run.