`forbidden.tsx` not being used when `forbidden()` was thrown in `layout.tsx`

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/73l4ht

To Reproduce

1. Add experimental: { authInterrupts: true } to next.config.js
2. Create some route
3. Add forbidden.tsx to this route
4. Add layout.tsx file to this route and throw forbidden() in it

Current vs. Expected behavior

Current behavior:
Next.js generic error page shows up with code 403.

--

Expected behavior:
forbidden.tsx content should be used instead.

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Pro
  Available memory (MB): 31913
  Available CPU cores: 24
Binaries:
  Node: 22.15.0
  npm: 11.5.2
  Yarn: 1.22.22
  pnpm: 10.14.0
Relevant Packages:
  next: 15.4.6 // There is a newer version (15.5.3) available, upgrade recommended! 
  eslint-config-next: 15.4.6
  react: 19.1.1
  react-dom: 19.1.1
  typescript: 5.8.3
Next.js Config:
  output: N/A

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

Not sure

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

next dev (local), next start (local)

Additional context

• Tested with 15.5.3 version of next.js. Behavior didn't change.
• Expected behavior happens if layout.tsx exists, but forbidden() is thrown on page.tsx

Start with the linked CodeSandbox and reproduce the issue using the route's layout.tsx, forbidden.tsx, and the authInterrupts setting in next.config.js. Compare behavior in next dev and next start; the work is done when throwing forbidden() from layout.tsx renders the route's forbidden.tsx content instead of the generic 403 page.