basePath not creating structure correctly when output set to export

[robmarshall]

Link to the code that reproduces this issue

https://github.com/robmarshall/next-output-export-reproduce

To Reproduce

1. Start application using Next Build
2. Serve the application (npx serve out)

Current vs. Expected behavior

The page.tsx file at root level should be returned at: "/app".
However it is only shown at "/" with all of its imports (styling/fav etc) broken - trying to find files at "/app".

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 10 Home
  Available memory (MB): 32653
  Available CPU cores: 8
Binaries:
  Node: 18.17.0
  npm: N/A
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 14.2.5 // Latest available version is detected (14.2.5).
  eslint-config-next: 14.2.5
  react: 18.3.1
  react-dom: 18.3.1
  typescript: 5.5.3
Next.js Config:
  output: export

Which area(s) are affected? (Select all that apply)

Output (export/standalone)

Which stage(s) are affected? (Select all that apply)

next dev (local), next build (local), next start (local), Vercel (Deployed)

Additional context

When removing the output key from the Next config, and building as a standard Next app, the routing works correctly.

Next Config:

const nextConfig = {
  images: {
    unoptimized: true,
  },
  basePath: "/app",
  trailingSlash: true,
  output: "export",
};