CSS prioritization differs between next dev --turbopack and next build --turbopack

[GlebKodrik]

Link to the code that reproduces this issue
https://github.com/GlebKodrik/turbopack-style/

To Reproduce

Run the app in development with next dev --turbopack
→ Styles load correctly, no issues observed.

Build the app with next build --turbopack and then start with next start
→ Styles are applied in a different priority/order compared to dev mode.

Notes:

The issue reproduces 100% on our current setup.
Sometimes deleting the pages folder or a random component in entities makes the bug disappear, which makes the behavior unpredictable.

Additional context
Happens locally with next build --turbopack → next start.
Cannot reproduce when running next dev --turbopack.
Issue sometimes disappears after removing a random folder/component, which suggests non-deterministic style bundling.

I’ve minimized the code a lot, so now it’s much easier to understand what’s going on. Please pay special attention to the page.

Reproduction link: http://localhost:3000/intensive

We are working with (pages).

If I delete the course page, the bug occurs.
If I move the block into the parent (page), the bug occurs.
Same thing with the intensive (page).
If I delete , the bug occurs.
If I move the content of directly into page, the bug also occurs.
Additional

If I delete a component that is not used anywhere at all (), the bug still occurs
Something very strange is happening here.