workflow:rules variables can not access passed variables from upstream and trigger variables

Summary

workflow:rules (as described in https://docs.gitlab.com/12.7/ee/ci/yaml/README.html#workflowrules) does not work properly with triggered pipelines and passing variables.

Steps to reproduce

Create upstream pipeline:

test multiproject:
  trigger: workflow/upstream
  variables:
    MYVAR: examplevalue
    
test trigger api:
  script:
    - "curl -X POST -F token=<your token here> -F ref=master -F variables[MYVAR]=examplevalue https://gitlab.com/api/v4/projects/123/trigger/pipeline"

Create downstream pipeline:

workflow:
  rules:
    - if: '$MYVAR'
      when: always
    - when: never

job1:
  script:
    - echo $MYVAR

Example Project

Example projects on gitlab.com: https://gitlab.com/workflow5/downstream https://gitlab.com/workflow5/upstream

What is the current bug behavior?

Job 'test multiproject' will be stuck. Job 'test trigger api' will have '{"message":{"base":["Pipeline filtered out by workflow rules."]}}' in output and no pipeline triggered.

What is the expected correct behavior?

Downstream Pipeline is triggered properly with defined variables.

Output of checks

This bug happens on GitLab.com and self-hosted GitLab Enterprise 12.7.5.

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:         RedHatEnterpriseServer 7.7
Proxy:          http_proxy: http://1.2.3.4:8080
https_proxy: http://1.2.3.4:8080
no_proxy: localhost,127.0.0.1
Current User:   git
Using RVM:      no
Ruby Version:   2.6.5p114
Gem Version:    2.7.10
Bundler Version:1.17.3
Rake Version:   12.3.3
Redis Version:  5.0.7
Git Version:    2.24.1
Sidekiq Version:5.2.7
Go Version:     unknown
GitLab information
Version:        12.7.5-ee
Revision:       19edff260da
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     PostgreSQL
DB Version:     10.9
URL:            https://sanitazed.com
HTTP Clone URL: https://sanitazed.com/some-group/some-project.git
SSH Clone URL:  sanitazed@sanitazed.com:some-group/some-project.git
Elasticsearch:  no
Geo:            no
Using LDAP:     yes
Using Omniauth: yes
Omniauth Providers: saml
GitLab Shell
Version:        11.0.0
Repository storage paths:

• default: /var/opt/gitlab/git-data/repositories GitLab Shell path: /opt/gitlab/embedded/service/gitlab-shell Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

Checking GitLab subtasks ...
Checking GitLab Shell ...
GitLab Shell: ... GitLab Shell version >= 11.0.0 ? ... OK (11.0.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Internal API available: OK
Redis available via internal API: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Gitaly ...
Gitaly: ... default ... OK
Checking Gitaly ... Finished
Checking Sidekiq ...
Sidekiq: ... Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Incoming Email ...
Incoming Email: ... Reply by email is disabled in config/gitlab.yml
Checking Incoming Email ... Finished
Checking LDAP ...
LDAP: ... Server: ldapmain
not verifying SSL hostname of LDAPS server 'sanitazed.com:636'
LDAP authentication... Success
LDAP users with access to your GitLab server (only showing the first 100 results)
not verifying SSL hostname of LDAPS server 'sanitazed.com:636'
not verifying SSL hostname of LDAPS server 'sanitazed.com:636'
User output sanitized. Found 44 users of 100 limit.
Checking LDAP ... Finished
Checking GitLab App ...
Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... yes
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
1/1 ... yes
3/4 ... yes
13/6 ... yes
13/7 ... yes
17/8 ... yes
15/11 ... yes
15/12 ... yes
23/13 ... yes
23/14 ... yes
23/15 ... yes
23/16 ... yes
25/18 ... yes
23/19 ... yes
23/21 ... yes
23/22 ... yes
15/24 ... yes
23/25 ... yes
13/26 ... yes
13/27 ... yes
15/29 ... yes
35/30 ... yes
15/31 ... yes
27/32 ... yes
23/33 ... yes
23/34 ... yes
15/37 ... yes
13/38 ... yes
15/39 ... yes
15/40 ... yes
13/41 ... yes
4/42 ... yes
13/43 ... yes
13/44 ... yes
50/45 ... yes
27/46 ... yes
27/47 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.5)
Git version >= 2.22.0 ? ... yes (2.24.1)
Git user has default SSH configuration? ... yes
Active users: ... 7
Is authorized keys file accessible? ... yes
Elasticsearch version 5.6 - 6.x? ... skipped (elasticsearch is disabled)
Checking GitLab App ... Finished
Checking GitLab subtasks ... Finished