Suggested change block is not rendered properly for pending comments

Summary

When I have a pending comment with a suggested change, rather than being rendered as a "Suggested change" with a highlighted diff, it is rendered as a simple code block.

Steps to reproduce

1. Navigate to a merge request and click the "Changes" tab
2. Click to comment on any line of code
3. Click the "Insert suggestion" button
4. Make a change
5. Click "Preview" to see the rendered diff
6. Click "Start a review"
7. Observe the rendered fenced code block, rather than the rendered diff

Example Project

An example project doesn't really show the repro (see screen recording below), but here is the merge request I used to create that recording:

https://gitlab.com/brett.higgins/mr-squash-test/-/merge_requests/2/diffs#note_371931275

(ignore the project name; I've used it for a couple different bug repros now)

What is the current bug behavior?

The rendered diff doesn't appear when the comment that contains it is pending

What is the expected correct behavior?

The rendered diff should appear in pending comments as well as submitted comments

Relevant logs and/or screenshots

Screen_Recording_2020-07-01_at_1.38.55_PM

Output of checks

This bug happens on GitLab.com