Upgrade from @mdx-js/react 1.6.19 to 1.6.20/1.6.21 breaks my syntax highlighting on Docusaurus v2

[HonkingGoose]

Subject of the issue

I was prompted to upgrade to @mdx-js/react v1.6.21 via the Renovate bot.
When I looked at my preview on Netlify, I noticed that my syntax highlighting is now broken where it worked before.
I manually installed @mdx-js/react v1.6.20 and that version also has this bug.

Things are working fine on version 1.6.19.

Your environment

• OS: Ubuntu 20.04
• Packages: See https://github.com/RoostingGeese/git-gosling/pull/82/files
• Env: Node v12.19.0 Yarn 1.22.5, Firefox 82.0.2 (64 bit)

Steps to reproduce

Tell us how to reproduce this issue. Please provide a working and simplified example.

This is working fine: https://git-gosling.netlify.app/docs/best_practices/split_up_your_commits
The pull request preview for mdx-js/react 1.6.21 shows: https://deploy-preview-82--git-gosling.netlify.app/docs/best_practices/split_up_your_commits

1. Clone https://github.com/RoostingGeese/git-gosling
2. Stay on the main branch.
3. git checkout c29382e to checkout the commit before I made further changes.
4. git switch -c reproduction_branch to create a branch to work on.
5. yarn install to install set of working dependencies.
6. yarn start to make a build and open a browser window
7. Click on the "docs` section in the top navbar.
8. Go to "Best practices - > Split up your commits" section of the web page, see black code block background, and red/green colors.
9. Stop dev server with CTRL + C in the CLI.
10. Run yarn upgrade-interactive --latest, and select the @mdx-js/react package with the spacebar, and confirm with enter.
11. Run yarn start again, make a dev build with the updated version.
12. Go to "Best practices - > Split up your commits" section, see gray code block background, syntax highlighting gone.

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Expected behavior

What should happen?
Syntax highlighting style stays the same when upgrading.

Actual behavior

What happens instead?

Syntax highlighting is broken.

[HonkingGoose]

I managed to fix this somehow by updating other dependencies, see HonkingGoose/git-gosling#85 for the changes I made that got it working again.

I have no clue what the real underlying problem is nor what package caused the problem. 😕
At least it's fixed now. 🎉

Closing this issue as my problem cannot be cleanly reproduced.