Sourcemaps are not loaded in index pages with a dynamic route level deeper than one

[albertodeago]

Link to the code that reproduces this issue

https://github.com/albertodeago/next-sourcemaps-repro

To Reproduce

Steps described also in the readme of the reproduction repo

Clone the reproduction repo and:

npm install
npm run build
npm run start

Open the app in the browser (localhost:3000), open the devtools and navigate through the pages.
Open (in the source panel) the sourcemaps relative to the /content/[subject]/[method]/index.page.tsx page, you will see that the sourcemaps are not there (chrome will show a 404 error).

The chunk is actually generated by next build but seems like it's not picked up by the server (my guess)

This happens to all the index.page.tsx pages with a dynamic route ([whatever]) with a level deeper than 1 (so /content/[subject]/index.page.tsx has the sourcemaps visible in the browser).

Note that it doesn't happen in a page that has a name different than index (e.g. /content/[subject]/[method]/named.page.tsx)

Current vs. Expected behavior

Current behaviour

sourcemaps on index pages with a dynamic route level deeper than 1 are not loaded

Expected behaviour

Any page sourcemap should be loaded in the browser

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.5.0: Wed May  1 20:14:38 PDT 2024; root:xnu-10063.121.3~5/RELEASE_ARM64_T6020
  Available memory (MB): 16384
  Available CPU cores: 12
Binaries:
  Node: 20.8.0
  npm: 10.1.0
  Yarn: 1.22.21
  pnpm: 8.8.0
Relevant Packages:
  next: 14.2.3 // Latest available version is detected (14.2.3).
  eslint-config-next: N/A
  react: 18.3.1
  react-dom: 18.3.1
  typescript: 5.4.5
Next.js Config:
  output: N/A

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

Not sure

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

next build (local), next start (local)

Additional context

• I tested this in multiple browsers (safari, chrome and firefox (all latest versions))

• Tried with both pageExtensions and without it and seems to affect both

• I did not test it with app router because this affect an application I'm working on that cannot migrate to app router yet

• The bug seems to affect only index pages (not pages with a different name), as it's described in the reproduction repo