Middleware source map is missing when in standalone mode

[seanparmelee]

Link to the code that reproduces this issue

https://github.com/seanparmelee/nextjs-middleware-source-map

To Reproduce

1. Run npm run build
2. Examine the .next/standalone/.next/server/ directory and note that middleware.js.map does not exist.
3. Examine the .next/server/ directory and note that middleware.js.map does exist.

Current vs. Expected behavior

Expected Behavior

Given an application that's using middleware and built in standalone mode, I'd expect to find a .next/standalone/.next/server/middleware.js.map file as that's where the sourceMappingURL in .next/standalone/.next/server/middleware.js file is referencing.

Current Behavior

The .next/standalone/.next/server/middleware.js.map file does not exisst, but there is .next/server/middleware.js.map.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.3.0: Wed Dec 20 21:30:44 PST 2023; root:xnu-10002.81.5~7/RELEASE_ARM64_T6000
  Available memory (MB): 32768
  Available CPU cores: 10
Binaries:
  Node: 20.11.0
  npm: 10.2.4
  Yarn: 1.22.19
  pnpm: 8.15.2
Relevant Packages:
  next: 14.2.0-canary.43 // Latest available version is detected (14.2.0-canary.43).
  eslint-config-next: N/A
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.1.3
Next.js Config:
  output: standalone

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

Middleware / Edge (API routes, runtime)

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

next build (local)

Additional context

We're ultimately copying the .next/standalone folder into a docker image and using @sentry/nextjs to upload the source maps so they can be incorporated into our Sentry issues. At the moment, it cannot find the source map for the middleware.ts file but I suspect we can workaround this in the meantime if we manually copy over the one from .next/server/.