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

Running multi-instance in <= v3.1.1 works flawlessly. Upgrading to >= v3.2.0 breaks the non-default instance and displays a "Page Not Found". This occurs when the default instance has a routeBasePath: '/'.

Reproducible demo

https://stackblitz.com/edit/github-cwn3ck?file=package.json

Steps to reproduce

1. Go to https://stackblitz.com/edit/github-cwn3ck?file=package.json
2. Notice that @docusaurus/core, @docusaurus/preset-classic, @docusaurus/module-type-aliases, @docusaurus/types are all at v3.1.1 in the package.json
3. Using the main navigation on the website, click on the Default Instance and Second Instance links. Both pages should appear as expected.
4. In the package.json file, update @docusaurus/core, @docusaurus/preset-classic, @docusaurus/module-type-aliases, @docusaurus/types to v3.2.1
5. Run the following commands in the terminal:

• Ctrl+C
• yarn clear
• yarn
• yarn start

6. Using the main navigation on the website, click on the Default Instance and Second Instance links. The Default Instance should appear as expected, but the Second Instance will display a Page Not Found.

Expected behavior

Both instances should work in v3.2.1

Actual behavior

The Default Instance appears as expected, but the Second Instance will display a Page Not Found.

Your environment

• Public source code: https://stackblitz.com/edit/github-cwn3ck?file=package.json
• Public site URL: https://stackblitz.com/edit/github-cwn3ck?file=package.json
• Docusaurus version used: 3.1.1 and 3.2.1
• Environment name and version (e.g. Chrome 89, Node.js 16.4): Chrome 123, Node 18
• Operating system and version (e.g. Ubuntu 20.04.2 LTS): Mac OS Ventura

Self-service

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