next/dynamic from external package causes hydration errors for combination of pages and app routers

Link to the code that reproduces this issue

https://github.com/adanperez/nextjs-dynamic-hydration-error

To Reproduce

• pnpm run build
• pnpm run start
• Go to http://localhost:3000/ and refresh the page a couple times
  You will see the following Hydration errors in the console.

framework-5e252d5045bb7a0e.js:9 Uncaught Error: Minified React error #418; visit https://reactjs.org/docs/error-decoder.html?invariant=418 for the full message or use the non-minified dev environment for full errors and additional helpful warnings.
  at ly (framework-5e252d5045bb7a0e.js:9:46791)
  at framework-5e252d5045bb7a0e.js:9:99911
  at oD (framework-5e252d5045bb7a0e.js:9:106131)
  at oO (framework-5e252d5045bb7a0e.js:9:99079)
  at framework-5e252d5045bb7a0e.js:9:98886
  at oF (framework-5e252d5045bb7a0e.js:9:98893)
  at oS (framework-5e252d5045bb7a0e.js:9:93932)
  at x (framework-5e252d5045bb7a0e.js:33:1364)
  at MessagePort.T (framework-5e252d5045bb7a0e.js:33:1894)

framework-5e252d5045bb7a0e.js:9 Uncaught Error: Minified React error #423; visit https://reactjs.org/docs/error-decoder.html?invariant=423 for the full message or use the non-minified dev environment for full errors and additional helpful warnings.
  at i (framework-5e252d5045bb7a0e.js:9:120721)
  at oO (framework-5e252d5045bb7a0e.js:9:99019)
  at framework-5e252d5045bb7a0e.js:9:98886
  at oF (framework-5e252d5045bb7a0e.js:9:98893)
  at ox (framework-5e252d5045bb7a0e.js:9:95645)
  at oS (framework-5e252d5045bb7a0e.js:9:94200)
  at x (framework-5e252d5045bb7a0e.js:33:1364)
  at MessagePort.T (framework-5e252d5045bb7a0e.js:33:1894)

Current vs. Expected behavior

Current

Currently we get hydration errors when using a next/dynamic component from an external package.

Expected

We do not expect to get hydration errors.

If you remove the app directory and rebuild and start the app, you will no longer see hydration errors. The combination of pages and app directories causes the hydration error.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.6.0: Mon Jul 29 21:14:30 PDT 2024; root:xnu-10063.141.2~1/RELEASE_ARM64_T6000
  Available memory (MB): 32768
  Available CPU cores: 10
Binaries:
  Node: 20.10.0
  npm: 10.2.3
  Yarn: N/A
  pnpm: 9.4.0
Relevant Packages:
  next: 14.2.5 // Latest available version is detected (14.2.5).
  eslint-config-next: 14.2.5
  react: 18.3.1
  react-dom: 18.3.1
  typescript: 5.5.4
Next.js Config:
  output: N/A

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

Lazy Loading, Pages Router, Runtime

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

next build (local), next start (local)

Additional context

No response

Clone the linked reproduction, run pnpm run build and pnpm run start, then refresh http://localhost:3000/ to confirm the hydration errors. Compare the result with the app directory removed; done means the pages-and-app-router combination with an external next/dynamic component no longer produces hydration errors.