GitLab CI job does not fail on non-zero exit code

Summary

Consider this .gitlab-ci.yml:

test:
  script:
    - false && true
    - echo why???

Which passes (see example), despite false && true exits with non-zero exit code. This can be observed by running below lines in any reasonable shell:

$ false && true
$ echo $?
1

This behaviour has been previously reported in gitlab-foss#20731 (comment 21596358).

Steps to reproduce

Example above should be pretty self-contained. Also see example project below.

Example Project

https://gitlab.com/yaroslav.admin/gitlab-ci-bug

What is the current bug behavior?

Job passes despite command exiting with non-zero exit code.

What is the expected correct behavior?

Job fails when command exits with non-zero exit code.

Relevant logs and/or screenshots

See job logs.

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)