dynamic route, dynamicIO, mdx, notFound => NEXT_HTTP_ERROR_FALLBACK;404

[volfadar]

Link to the code that reproduces this issue

https://github.com/volfadar/next-bug

To Reproduce

1. bun install
2. bun dev
3. go to "http://localhost:3000/lecture/marketing/intro"

Current vs. Expected behavior

Simply, it should not throw an error NEXT_HTTP_ERROR_FALLBACK;404 because I already have a not-found.tsx.

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Home Single Language
  Available memory (MB): 16087
  Available CPU cores: 8
Binaries:
  Node: 20.11.1
  npm: 10.2.4
  Yarn: 1.22.22
  pnpm: 9.0.2
Relevant Packages:
  next: 15.2.0-canary.15 // Latest available version is detected (15.2.0-canary.15).
  eslint-config-next: N/A
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.7.3
Next.js Config:
  output: N/A

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

dynamicIO, Turbopack, TypeScript, Markdown (MDX)

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

next dev (local)

Additional context

No response

[volfadar]

related issue #73130 & pr #73210