This is a bug.
The Bulma Sass library leverages directory index files with its @use and @forward rules.
Directory index files allow you to do this…
@use "foo/bar";
…instead of either of these:
@use "foo/bar/_index";
@use "foo/bar/_index.scss";
When you install the Bulma Sass library using npm install or similar, its files exist in the OS file system. In this case the Dart Sass executable is able to resolve paths to directory index files as expected.
When you import the Bulma Sass library as a Hugo module, its files exist in Hugo’s virtual file system. In this case the Dart Sass executable is unable to resolve paths to directory index files because the files do not exist in the OS file system.
This should be fixable. See:
Minimal reproducible example:
git clone --single-branch -b hugo-forum-topic-51528 https://github.com/jmooring/hugo-testing hugo-forum-topic-51528
cd hugo-forum-topic-51528
hugo server