@source directive not working in Stackblitz when targeting node_modules

[Powerplex]

What version of Tailwind CSS are you using?

4.1.11

What build tool (or framework if it abstracts the build tool) are you using?

"postcss": "8.5.3", "@tailwindcss/postcss": "^4.1.11",

What version of Node.js are you using?

Node v20.19.1

What browser are you using?

Chrome

What operating system are you using?

macOS

Reproduction URL

https://stackblitz.com/edit/stackblitz-starters-wu3sn9fa?file=tailwind.css

Describe your issue

A few months ago this issue was fixed that allowed to use Tailwind v4 in Stackblitz: windlabs/tailwindcss/issues/13133

Unfortunately, it only works for a basic/default tailwind config and comes with a limitation: I cannot parse my design system library (a dependency) using @source (@source './node_modules/@spark-ui';).

The styles are not there for some reason. It has been a pain point for us this whole year because we were using Stackblitz as a bug reproduction environment for our users, and now we simply have no mean to do so (We had no issue with v3).