CI_COMMIT_REF_SLUG variable don't contains expected branch name during MERGE REQUESTS

Summary

Durring merge-request, CI_COMMIT_REF_SLUG variable do not contains the branch name.
We've been using this value for the last 6months. The variable content changed durring the last 2weeks.

Steps to reproduce

create a job with only : [merge_requests]
Simply call a script with echo branch = $CI_COMMIT_REF_SLUG

Example Project

https://gitlab.com/gitlab-cd/sandbox/tree/invalid-ref-slug

What is the current bug behavior?

display a kind of merge request mnemonic "refs-branch-name-4-head"

What is the expected correct behavior?

should simply dispalay branch name "branch-name"

Relevant logs and/or screenshots

Checking out 2482279f as refs/merge-requests/4/head...

Skipping Git submodules setup
$ echo CI_COMMIT_REF_SLUG = $CI_COMMIT_REF_SLUG
CI_COMMIT_REF_SLUG = refs-merge-requests-4-head
Job succeeded

Output of checks

This bug happens on GitLab.com

Possible fixes

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