"Don't show me this again" button broken (Epic)

Summary

"Don't show me this again" button does nothing when on an issue (rightmost panel) in the "Epic" section.

Steps to reproduce

(Tested with Firefox and Opera)

• Clear you browser cache and cookies, or open a "private navigation" tab.
• Login and go to an issue URL.
• In the right panel > Epic > Upgrade Plan, click "Don't show me this again".
• Reloading the page doesn't work either.

What is the expected correct behavior?

The "Epic" section should be hidden, and choice should be remembered across sessions (as for "Weight").

Output of checks

• This bug happens on GitLab.com.
• Cookie promote_epics_sidebar_dismissed is set to true, but this doesn't seem to change a thing.