Returning `notFound` from `getStaticProps` with dynamic path and middleware shows error page instead of 404 page

### Link to the code that reproduces this issue https://codesandbox.io/p/devbox/dreamy-villani-mh2lrd?workspaceId=ws_PaH83X1dr2DdF1ajuypJkh ### To Reproduce 1. Start the application in development mode (next dev) 2. Navigate to the dynamic path, for instance: https://mh2lrd-3001.csb.app/products/3/hello 3. The following error is shown: `Error: Failed to load static props` ### Current vs. Expected behavior **Expected:** When navigating to a route with `getStaticProps` that returns `notFound: true`, I expect to see the custom 404 page. **Actual:** Instead, the default error page is shown. If the middleware is removed, the bug does not reproduce - the 404 page is shown. From the network calls, it appears that a fetch to 404.json with query params representing the dymanic path fails (with a 404 status code). For instance - a call to: `_next/data/cYcaBxmaNOQIDHRyOqEZf/404.json?id=3&slug=hello` has a 404 response. ### Provide environment information ```bash Operating System: Platform: linux Arch: x64 Version: #1 SMP PREEMPT_DYNAMIC Sun Aug 6 20:05:33 UTC 2023 Available memory (MB): 4102 Available CPU cores: 2 Binaries: Node: 20.9.0 npm: 9.8.1 Yarn: 1.22.19 pnpm: 8.10.2 Relevant Packages: next: 15.1.1-canary.18 // Latest available version is detected (15.1.1-canary.18). eslint-config-next: N/A react: 19.0.0-beta-04b058868c-20240508 react-dom: 19.0.0-beta-04b058868c-20240508 typescript: 5.1.3 Next.js Config: output: N/A ``` ### Which area(s) are affected? (Select all that apply) Middleware, Pages Router ### Which stage(s) are affected? (Select all that apply) next dev (local), next start (local), Other (Deployed) ### Additional context The bug does not reproduce in next@14.1.4. Any version from next@14.2.0 onwards reproduces the bug. It reproduces in dev, running locally, and in our production environment (self hosted).