"Add comment now" or submit review on existing diff review doesn't show the comment

Summary

If I'm replying to an existing diff discussion and I submit a review or start a review and then click "Add comment now" the comment doesn't appear unless I refresh the page.

Steps to reproduce

1. Start a discussion on a diff line.
2. Add a comment to the discussion and hit "Start a review".
3. Click "Add comment now" or submit the review.

Example Project

Any MR on gitlab.com. E.g.: mlapierre/test!5 (diffs)

What is the current bug behavior?

The comment disappears. You have to refresh the page to see it.

What is the expected correct behavior?

The comment appears

Output of checks

This bug happens on GitLab.com

/cc @jramsay @andr3 @iamphill