conditional job causes an unwanted and broken merge request pipeline

Summary

When a job contains rules, it will cause an merge pipeline to be spawned.

Steps to reproduce

1. Create a repository with a .gitlab-ci.yml file:

   # build job creates artifact
   build:
     script:
       - echo "Jussomething" > artifact.txt
     artifacts:
       paths:
         - artifact.txt
   
   # deploy job uses artifact
   deploy:onmyserver:
     stage: deploy
     script:
       - cat artifact.txt
     rules:
       # Would normally be some TOKEN for authentification
       - if: $CI_COMMIT_REF_SLUG

2. Push this to the default branch

3. Make any simple change, push a new merge request:

   git push -o merge_request.create local HEAD:newbranch

Example Project

Reproduce Bug Unwanted Merge Pipeline.

What is the current bug behavior?

There will be 2 pipelines created for the merge request:

1. One for the branch/push (which is okay)
2. One detached merge request pipeline, which only runs the jobs that have a matched rule

What is the expected correct behavior?

From the documentation, as far as I understand it, only 1 pipeline should run.

Relevant logs and/or screenshots

see the live variant here.

Output of checks

This bug happens on GitLab.com