I asked LearnDash is there was anything specific in your theme that could be causing focus mode to not work and their reply was:
It’s hard to say, but I would start by looking to see if there’s a JavaScript conflict. Sometimes, importing a script incorrectly on a page can cause this.
If they find that there is code change needed on our part, we’re happy to look into it further and see if that’s possible.
Please let me know if you are able to find out what in the theme is causing the issue with quizzes and focus mode and I can pass it along to LearnDash, in case others experience the same issue.