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

[yoniheib]

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

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 [email protected].
Any version from [email protected] onwards reproduces the bug.

It reproduces in dev, running locally, and in our production environment (self hosted).