Questions which do not have any tags associated with any languages do not get their code blocks automatically highlighted at all. Examples:
https://stackoverflow.com/questions/61112840/preloader-is-not-working-on-angular-universal-ssr-app
How to Get a List of Members in a Guild Discord.js
Note that questions will get highlighted if they have at least one tag with a "Highlight Language" in their wiki, even if that language is default - like with regex. Questions with at least one such tag will get their code blocks automatically highlighted. In contrast, a question only with tags like discord.js which has no highlight language (not even default) will not have any code blocks highlighted.
I think when no tags have languages associated with them, the question's code blocks should be highlighted automatically. Maybe remove the difference between the association with default highlighting and a non-existing language association while you're at it, unless it's needed for something. (Or give all tags a default language association.)
All questions should at least have something like
<div style="display:none" id="js-codeblock-lang">default</div>
but it should never be empty, or auto-highlighting won't work:
<div style="display:none" id="js-codeblock-lang"></div>
This issue is pretty similar to a related standalone question: Improving syntax highlighting language auto-detection.