[Turbopack | CSS]: CSS prioritization differs between next dev --turbopack and next build --turbopack

[SashaPolyanski]

Link to the code that reproduces this issue

https://github.com/SashaPolyanski/next-turbopack-style

To Reproduce

Use route my project with problem: http://localhost:3000/intensive or https://next-turbopack-style-edr3.vercel.app/intensive

To Reproduce

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

2. 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.

Current vs. Expected behavior

• Current: In production (next build --turbopack + next start), CSS order/priority differs from development, breaking the layout.
• Expected: Style prioritization should be consistent between dev and build modes when using Turbopack.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.6.0
Binaries:
  Node: 20.9.0
  npm: 10.8.2
Relevant Packages:
  next: 15.6.0-canary.12
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Turbopack

Which stage(s) are affected? (Select all that apply)

next build (local)

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.

1 image - next dev --turbopack
2 image next build --turbopack → next start