Successful pipeline notification is always sent

Summary

I have my notification level set to Custom with the "Successful pipeline" unchecked, but every time I push a new commit and the pipeline succeeds, I still get a notification email.

Steps to reproduce

In a project, select Custom notification and turn all notifications on except for "Successful pipeline".

Expected behavior

When the pipeline succeeds, nothing should happen.

Actual behavior

When the pipeline succeeds, I receive a notification email.

Output of checks

This bug happens on GitLab.com