The chunks of the parent page are loaded in the child page

### Link to the code that reproduces this issue https://github.com/noreiller/next-preload-parent-page-bug ### To Reproduce First, run the development server: ```bash npm run dev # or yarn dev # or pnpm dev # or bun dev ``` Open [http://localhost:3000](http://localhost:3000) with your browser to load the parent page. Please open the Network tab of the browser Dev Tools and click on the `go to child page` link. Once, the page is loaded, please refresh the page to force the full server side render. You should be able to see the following result: the 2 `page.js` chunks of the current and the parent pages are loaded. ### Current vs. Expected behavior Following the steps from the previous section, I expected that only the chunks of the layout and current page would be loaded. ### Provide environment information ```bash Operating System: Platform: linux Arch: x64 Version: #29-Ubuntu SMP PREEMPT_DYNAMIC Fri Jun 13 20:29:41 UTC 2025 Available memory (MB): 14873 Available CPU cores: 8 Binaries: Node: 18.18.2 npm: 9.8.1 Yarn: 1.22.22 pnpm: 8.2.0 Relevant Packages: next: 15.4.7 // Latest available version is detected (15.4.7). eslint-config-next: 15.4.7 react: 19.1.0 react-dom: 19.1.0 typescript: 5.8.2 Next.js Config: output: N/A ``` ### Which area(s) are affected? (Select all that apply) Webpack, Performance ### Which stage(s) are affected? (Select all that apply) next dev (local), next build (local), next start (local), Other (Deployed) ### Additional context I investigated a lot to find the origin of the bug and I found that it's the `ClientReferenceManifestPlugin` [webpack plugin](https://github.com/vercel/next.js/blob/v15.4.7/packages/next/src/build/webpack/plugins/flight-manifest-plugin.ts) which is faulty. Once the page manifest is generated, it's associated to a group which does not consider that when a module is used both in a `layout.js` and a `page.js` file in the same segemnt level, the `layout.js` manifest has more priority than the `p…