Fast Refresh Performs Full Reload on Pages Starting With `%5F` (Underscore)

### Link to the code that reproduces this issue https://github.com/jakub300/nextjs-bug-underscore-fast-refresh ### To Reproduce 1. Start the application in development 1. Go to `/_example` 1. Modify `some-client-component.tsx` ### Current vs. Expected behavior Following the steps from the previous section, I expected component reload, but page reloads instead. ### Provide environment information ```bash Operating System: Platform: win32 Arch: x64 Version: Windows 10 Pro Available memory (MB): 32689 Available CPU cores: 16 Binaries: Node: 22.17.0 npm: 10.9.2 Yarn: 1.22.22 pnpm: N/A Relevant Packages: next: 15.4.2-canary.51 // Latest available version is detected (15.4.2-canary.51). eslint-config-next: N/A react: 19.1.1 react-dom: 19.1.1 typescript: 5.9.2 Next.js Config: output: N/A ``` ### Which area(s) are affected? (Select all that apply) Webpack, Linking and Navigating ### Which stage(s) are affected? (Select all that apply) next dev (local) ### Additional context Works correctly with Turbopack. Video demonstrating the bug: https://github.com/user-attachments/assets/e994e589-76a8-434d-80d5-7a751493fbdd