Build fails when blog is a subdirectory of the docs

[Salamandar]

Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

For corporate reasons™ I need to store the blog posts in a subdirectory of the docs (actually, in my specific case, docs path = '..', and blog path = '../release_notes', but I can reproduce the bug without '..' in the paths for a more standard installation).

The build fails with a kinda weird error.

Reproducible demo

No response

Steps to reproduce

npx create-docusaurus@latest my-website classic --typescript
cd my-website
mv blog docs

Then patch docusaurus.config.ts:

• Add the line exclude: ['blog'], in the docs preset
• Add the line path: 'docs/blog', in the blog preset

Then yarn build.

Expected behavior

A perfectly indistinguishable build from the example

Actual behavior

A bunch of those errors:

lient bundle compiled with errors therefore further build is impossible.
Error: MDX compilation failed for file "/home/felix/tmp/my-website/docs/blog/2019-05-28-first-blog-post.md"
Cause: Unexpected `FunctionDeclaration` in code: only import/exports are supported
Details:
{
  "column": 1,
  "file": "",
  "message": "Unexpected `FunctionDeclaration` in code: only import/exports are supported",
  "line": 24,
  "name": "24:1-33:2",
  "place": {
    "start": {
      "line": 24,
      "column": 1,
      "offset": 564
    },
    "end": {
      "line": 33,
      "column": 2,
      "offset": 778
    }
  },
  "reason": "Unexpected `FunctionDeclaration` in code: only import/exports are supported",
  "ruleId": "non-esm",
  "source": "micromark-extension-mdxjs-esm",
  "url": "https://github.com/micromark/micromark-extension-mdxjs-esm#unexpected-type-in-code-only-importexports-are-supported"
}

--------------------------

Your environment

• Docusaurus version used: 3.9.2
• Environment name and version (e.g. Chrome 89, Node.js 16.4): Nodejs 22.21.0
• Operating system and version (e.g. Ubuntu 20.04.2 LTS): Ubuntu 24.04

Self-service

• I'd be willing to fix this bug myself.