Some CSS declarations are just *missing*

Probably not.

Only if whatever you’re working with has equivalent build and preview scripts in the package.json

This seems to be the equivalent for Create React App as per:
https://create-react-app.dev/docs/deployment/

There could be many causes, but the important part is that you know Netlify isn’t causing it, so you can proceed with searching for a solution and debugging your project locally.

Try things like:
https://www.google.com/search?q=create+react+app+production+build+missing+css
https://answers.netlify.com/t/some-css-styles-not-work-on-deployment/92506/6