Customized 404 not applied for docs/ sub routes

### Have you read the Contributing Guidelines on issues?

- [X] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#issues).

### Prerequisites

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

### Description

Follow-up of discussion [#9664](https://github.com/facebook/docusaurus/discussions/9664).

It seems that swizzled 404 are resolved when a root URL is accessed but, not when a sub-route is loaded.

e.g.

`http://localhost:3000/unknown` -> custom 404
`http://localhost:3000/docs/unknown` -> default 404 (or custom 404 for a milli seconds and default afterwards)

### Reproducible demo

_No response_

### Steps to reproduce

git clone https://github.com/peterpeterparker/docusaurus-404
cd docusaurus-404
npm ci
npm run start

try various unknown routes

### Expected behavior

Custom 404 displayed for any 404 regardless of the (sub)route.

### Actual behavior

As described in the details.

### Your environment

_No response_

### Self-service

- [ ] I'd be willing to fix this bug myself.