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

Link to the code that reproduces this issue

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

To Reproduce

1. npm install
2. npm run build
3. npm run start
4. Go to localhost:3000 and open the Network tab in your browser's dev tools
5. Refresh the page
6. 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

Start with the linked reproduction and next.config.ts at line 7, then run the npm install, build, and start steps against 16.1.7 and 16.2.x. Trace how the configured build ID reaches the _next/static and _next/data requests; done means generateBuildId is respected in both request paths as it was before 16.2.0.