"Preview changes" on New/Edit Release and New Snippet page doesn't work

Summary

On the New Release (or Edit release page), the "Preview changes" option in the Release notes field doesn't work:

This same behavior happens on the New Snippet page.

Steps to reproduce

1. Navigate a project's New Release page (Releases > New Release)
2. Type some text in the Release notes field
3. Click the Preview changes tab

What is the current bug behavior?

The preview always shows "Nothing to preview."

What is the expected correct behavior?

The preview shows a rendered version of the markdown.

Output of checks

This bug happens on GitLab.com.