With a virgin installation of Hugo v0.88.0, your equations will not render. I am not sure why it was working for you. It shouldn’t have, unless your theme/modules are doing something else in the background.
As to why it is not working:
https://discourse.gohugo.io/t//40998/4
Make sure to test the alternative:
https://discourse.gohugo.io/t//40998/#alternative-3
I’ve added your equations to that repository and it works fine.
The important files in that example:
- layouts/_default/baseof.html
- layouts/partials/katex.html
- layouts/shortcodes/katex.html
- layouts/_default/_markup/render-codeblock-katex.html