Launching a single locale with subpath does not work without hacks #31675

Command

build

Is this a regression?

• Yes, this behavior used to work in the previous version

The previous version in which this bug was not present was

No response

Description

Building a single locale which has a subpath results in Angular not being able to "find" the correct server to launch.

Minimal Reproduction

Link to reproduction repo: https://github.com/vzarskus/locale-manifest-issue

This repo has 2 locales: "en" on no subpath and "lt" on /lt subpath.

Scenario 1: running full build with both locales works:
Steps:

1. Run npm run build
2. Run npm run serve:ssr:locale-manifest-issue
3. Open it in the browser
4. Try changing url in url bar to /lt and back to /
5. Everything should work

Scenario 2: running just EN build works:
Steps:

1. Run npm run build:en
2. Run npm run serve:ssr:locale-manifest-issue
3. Open it in the browser
4. Works (EN page is displayed)

Scenario 3: running just LT build does not work:
Steps:

1. Run npm run build:lt
2. Run npm run serve:ssr:locale-manifest-issue
3. Open it in the browser
4. Does not work (LT page is not displayed neither on /lt nor on root url)

Looking at the compiled output of Scenario 3, I can find the culprit in dist/locale-manifest-issue/server/server.mjs getEntryPointExportsForUrl function:

getEntryPointExportsForUrl(t) {
  let { basePath: n } = this.manifest;
  if (this.supportedLocales.length === 1) return this.getEntryPointExports(''); // <---- Delete this line
  let r = cm(t, n);
  return this.getEntryPointExports(r) ?? this.getEntryPointExports('');
}

When there's a single locale, Angular always just assumes that the entrypoint is in server root. However, for LT locale, the entrypoint is actually in lt folder (dist/locale-manifest-issue/server/lt/main.server.mjs).
Deleting the line marked with "// <---- Delete this line" comment fixes the issue. Meaning that visiting /lt now works as expected.

Exception or Error

Your Environment

Node: 22.17.0
Package Manager: npm 11.4.2
OS: linux x64
    

Angular: 20.3.9
... common, compiler, compiler-cli, core, forms, localize
... platform-browser, platform-server, router

Package                      Version
------------------------------------
@angular-devkit/architect    0.2003.8
@angular-devkit/core         20.3.8
@angular-devkit/schematics   20.3.8
@angular/build               20.3.8
@angular/cli                 20.3.8
@angular/ssr                 20.3.8
@schematics/angular          20.3.8
rxjs                         7.8.2
typescript                   5.9.3

Anything else relevant?

No response