rules:if causes two pipelines to be run on MRs and jobs to be missing

Summary

When using rules:if when, after force pushing to MR, instead of 1 pipeline, 2 are run, one regular and one detached MR. To make things worse, the detached pipeline is mutilated, jobs that have no rules defined on them are missing.

This bug breaks MRs' pipelines when rules: are used.

This makes rules: rather hard to apply in complex pipelines with dependencies. Imagine there's a deploy job, that can be triggered manually (when: manual) on every pipeline (think of: deploy to demo lab) and should be triggered on a scheduled nightly pipeline (think of: bring demo lab up to speed) always (when: on_success). This deploy job depends on a build job, that has to run on every pipeline, a job that does not need rules.

Instead I'll end up with duplicating jobs and toying with only/except. Also I guess, spamming with rules: could also solve this issue.

Steps to reproduce

1. Have .gitlab-ci.yml with rules on at least one job and at least one job without it
2. optionally: to a job with rules, add a needs/depends to make it depend on a job without rules
3. Commit some changes
4. Make MR
5. Observe pipelines in MR

Example Project

https://gitlab.com/falxcerebri/gitlab-ci-rules-bug/

What is the current bug behavior?

When there's a rules: block present in .gitlab-ci.yml block, two pipelines are run, one regular, and one detached on top of it. The detached features only jobs with rules and nothing else, leading to pipeline failing on unmet dependencies.

falxcerebri/gitlab-ci-rules-bug!1

Note that rules: only change when from manual to on_success, not the actual presence of any job.

Everything is exposed in my sample repo: https://gitlab.com/falxcerebri/gitlab-ci-rules-bug/

What is the expected correct behavior?

1. There should be just one pipeline in Merge Request, since there's no only:/except: merge_request used anywhere
2. Even if 2 pipelines are necessary due some technical reasons, the resulting detached pipeline should consist of all jobs, not only these with rules: prop

Relevant logs and/or screenshots

falxcerebri/gitlab-ci-rules-bug!2

Output of checks

GitLab.com

Possible fixes

Unfortunately I have no idea. I guess this may be somewhat linked with detached pipeline settings but that's just a wild guess.