`generateBuildId` in `next.config.ts` not respected in v16.2.0+

Why this is a good first issue

The issue involves a regression in build ID generation affecting caching behavior.

AI Summary

The `generateBuildId` function in `next.config.ts` is not being respected in Next.js v16.2.0+, leading to non-unique build IDs across releases. This causes caching issues and 404 errors on initial page loads. The fix likely involves investigating the build system changes introduced in v16.2.0.

Issue Description

Link to the code that reproduces this issue

https://github.com/stayko-chalakov/build-id-issue-16.2

To Reproduce

npm install
npm run build
npm run start
Go to localhost:3000 and open the Network tab in your browser's dev tools
Refresh the page
Type build- in the filter input in Network tab and observe the requests to _/next/static and _next/data with build-TfctsWXpff2fKS

Current vs. Expected behavior

Current (16.2.0 or 16.2.1 or canary):

Expected (before 16.2.0, e.g 16.1.7):

generateBuildId here is not being respected: https://github.com/stayko-chalakov/build-id-issue-16.2/blob/b5ef22c929995fbc383aaca8ae8744f23ccecd81/next.config.ts#L7

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 25.0.0: Wed Sep 17 21:41:45 PDT 2025; root:xnu-12377.1.9~141/RELEASE_ARM64_T6000
  Available memory (MB): 16384
  Available CPU cores: 10
Binaries:
  Node: 23.11.0
  npm: 11.6.3
  Yarn: 1.22.22
  pnpm: 8.15.5
Relevant Packages:
  next: 16.1.7 // There is a newer version (16.2.1) available, upgrade recommended!
  eslint-config-next: N/A
  react: 19.2.4
  react-dom: 19.2.4
  typescript: 5.9.3
Next.js Config:
  output: N/A
⚠ There is a newer version (16.2.1) available, upgrade recommended!
   Please try the latest canary version (`npm install next@canary`) to confirm the issue still exists before creating a new issue.
   Read more - https://nextjs.org/docs/messages/opening-an-issue

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

Not sure

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

next start (local)

Additional context

No response