Expandable text in issue description is broken

Summary

Expandable text in issues descriptions is not rendered correctly. It shows raw MarkDown source code instead of formatted text. See the screenshot below.

Steps to reproduce

See issue: #3159 - List filter for merge requests that have not reached required number of approvals (Web and API)

Example Project

See https://gitlab.com/gitlab-org/gitlab-ee

What is the current bug behavior?

When the collapsed text gets expanded, it shows raw MarkDown text.

What is the expected correct behavior?

It it show rendered MarkDown.

Relevant logs and/or screenshots

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)