"Reopen merge request" button does nothing

Summary

The "Reopen merge request" button does nothing.

Steps to reproduce

1. Open a merged merge request.
2. Press the "Reopen merge request" button at the bottom.

The button changes into "Close merge request" but actually the merge request's status remains "Merged".

Example Project

tougouda/issue-demo!4 (merged)

What is the current bug behavior?

Nothing happens when clicking on the "Reopen merge request" button.

What is the expected correct behavior?

The merge request's status should changed into "Open".

Output of checks

This bug happens on GitLab.com