[Turbopack] [Pages] Hydration error (or double render) when using dynamic imports in _app.tsx

Link to the code that reproduces this issue

https://github.com/rtrembecky/next-turbopack-hydration-repro

To Reproduce

1. Install the deps - pnpm i
2. Start the app - pnpm dev
3. Navigate to http://localhost:3000

Current vs. Expected behavior

Current behavior:

Expected: No errors.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:29 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T6000
  Available memory (MB): 32768
  Available CPU cores: 10
Binaries:
  Node: 24.2.0
  npm: 11.4.0
  Yarn: 1.22.19
  pnpm: 9.15.9
Relevant Packages:
  next: 16.0.1-canary.2 // Latest available version is detected (16.0.1-canary.2).
  eslint-config-next: N/A
  react: 19.2.0
  react-dom: 19.2.0
  typescript: 5.9.3
Next.js Config:
  output: N/A

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

Pages Router, Module Resolution, Turbopack

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

next dev (local), next build (local), Other (Deployed)

Additional context

Pinpointed broken versions:

• canary: >= 15.1.1-canary.14, up to 16.0.1-canary.2 (latest at the time of writing)
• stable: >= 15.2.0

Working versions:

• 15.1.1-canary.13
• 15.1.6

Background:

• _app.tsx imports a component via next/dynamic - this causes a hydration issue
• when the same component is imported from the page level as well, it works - http://localhost:3000/works page (in repro repo) works
• when the <main> wrapper in _app.tsx is removed or replaced by a <> fragment, the dynamic element is rendered TWICE instead
  • so this issue looks related: https://github.com/vercel/next.js/issues/77712
  • and also this one where @gurkerl83 reported it's fixed in 15.1.1-canary.14, though it apparently broke something: https://github.com/vercel/next.js/issues/70795#issuecomment-2555971126
    • cc @timneutkens as you were involved on that one, but I understand it's been quite a while

Real world scenario:

• On www.groupon.com, we use const Layout = Component.getLayout || DefaultLayout; in _app.tsx, where DefaultLayout is imported dynamically - it's not needed for a user that accesses only pages with custom layouts (defined on the page level via Page.getLayout).
  • -> We are getting hydration errors in dev when using Turbopack, and also production React minified errors (even though we haven't migrated build to Turbopack yet, so it might be a different issue).

Start with the linked reproduction: run pnpm i, pnpm dev, and compare the root page with /works. Inspect the dynamic import in _app.tsx and the related Pages Router behavior; done means the root page has no hydration errors or duplicate rendering while /works continues to work.