loading.tsx not affecting sub-routes in some cases

[UltimateGG]

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/jovial-shockley-7yn8kf?workspaceId=ws_RG1nRyxTaV3usxvsfBToRH

To Reproduce

1. Click the link to go to the other route, notice it is delayed until the page appears
2. Click the link go to back home, it also will delay until home appears
3. If you manually refresh the page on either route, the Loading.tsx file will display properly

However, if you move the (groupA)/page.tsx up into the app/ folder, it works properly, including for the /other/[slug] route.

Current vs. Expected behavior

Current: loading.tsx only shows on page refresh, not client side navigations.
Expected: Loader should "trickle" down to the routes below.

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 10 Home
  Available memory (MB): 32703
  Available CPU cores: 8
Binaries:
  Node: 20.11.0
  npm: 9.8.1
  Yarn: N/A
  pnpm: 9.15.1
Relevant Packages:
  next: 15.1.2 // There is a newer version (15.1.3) available, upgrade recommended!
  eslint-config-next: 15.1.2
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.7.2
Next.js Config:
  output: N/A
 ⚠ There is a newer version (15.1.3) available, upgrade recommended!

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

Navigation

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

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

Additional context

CSB is on latest canary

[AliRazaDev27]

I have seen the same question asked many times before, Read the docs.
https://nextjs.org/docs/app/building-your-application/routing/loading-ui-and-streaming#instant-loading-states

[UltimateGG]

Just to confirm, it is intentional that loading does not affect routes below?