Checking a todo checkbox in issue body rewrites the issue content

Summary

Checking a checkbox in issue body rewrites the issue content. I just lost all my issue content =((

Steps to reproduce

1. Create a new issue.
2. Put a * [ ] Hey! I like to DELETE all the content as an issue body and save the issue.
3. Open a new tab in your browser (don't close the first one).
4. In the new tab open the issue, then click Edit button.
5. Add some cool stuff into the issue body.
6. Get back to the first tab and click to the checkbox.
7. Refresh the issue page. You will see that everything lost you added in the step 5.

Expected behavior

Only checked checkbox is changed. Everything else must stay as it is.

Actual behavior

Checkbox rewrites all the content.

Output of checks

GitLab Enterprise Edition 8.16.0-ee c13afce