Sourcemaps are inaccurate

### Link to the code that reproduces this issue https://github.com/vercel/next-app-router-playground ### To Reproduce 1. Install the latest canary version in https://github.com/vercel/next-app-router-playground project. 2. Edit `next.config.ts` to include sourcemaps using webpack: ```const nextConfig = { pageExtensions: ['js', 'jsx', 'mdx', 'ts', 'tsx'], experimental: { inlineCss: true, dynamicIO: true, clientSegmentCache: true, viewTransition: true, prerenderEarlyExit: false, routerBFCache: true, }, webpack: (config) => { config.devtool = 'source-map'; return config; }, } satisfies NextConfig; ``` 3. Run `pnpm build` 4. In `.next/server/app/context` you will find `page.js` and `page.js.map` 5. Use some kind of sourcemap resolver, I used https://evanw.github.io/source-map-visualization/ 6. On line 2, column 14664 you will see the parameter named `a` which should be resolved to `product`, but it doesn't resolve to a name (it almost points to the correct location in the sourcemap, but it doesn't have a name). <img width="1501" alt="Image" src="https://github.com/user-attachments/assets/2346ad38-2741-44cb-8ad9-8ca343efc43f" /> ### Current vs. Expected behavior Most of the sourcemaps are correct, but in some cases looks like they are off a little and aren't resolving to the actual value. In this case, the parameter should be resolved to `product` but it doesn't resolve to anything. There are more cases on multiple projects I have tested on, and more scenarios other than parameters (functions, objects, etc). If it's needed I can provide more examples. ### Provide environment information ```bash Operating System: Platform: darwin Arch: arm64 Version: Darwin Kernel Version 23.4.0: Fri Mar 15 00:10:42 PDT 2024; root:xnu-10063.101.17~1/RELEASE_ARM64_T6000 Available memory (MB): 32768 Available CPU cores: 10 Binaries: Node: 18.20.2 npm: 10.5.0 Yarn: 1.22.22 pnpm: 10.11.0 Relevant Packages: next: 15.4.0-canary.86 //…