@next/mdx: String format plugins not working with Turbopack and `mdxRs` flag enabled

[PlagueFPS]

Link to the code that reproduces this issue

https://github.com/PlagueFPS/next-turbopack-mdx-repro

To Reproduce

Steps to Reproduce

Clone and install:

git clone https://github.com/PlagueFPS/next-turbopack-mdx-repro
cd next-turbopack-mdx-repro
npm install

Run the dev server:

npm run dev

Open the app:
Visit http://localhost:3000.

Current vs. Expected behavior

Expected:
The table in test-content.mdx should render as an HTML table (since remark-gfm is enabled).

Actual:
The table renders as plain text, not as an HTML table.
This shows that remark-gfm is not applied.

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Home
  Available memory (MB): 32586
  Available CPU cores: 16
Binaries:
  Node: 22.18.0
  npm: 10.5.1
  Yarn: N/A
  pnpm: 10.14.0
Relevant Packages:
  next: 15.6.0-canary.31 // Latest available version is detected (15.6.0-canary.31).
  eslint-config-next: N/A
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.9.2
Next.js Config:
  output: N/A

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

Markdown (MDX), Turbopack

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

next dev (local), next build (local)

Additional context

No response