gatsby-plugin-mdx generates content but goes to 404 page

[JulSeb42]

Description

I'm trying to create a blog with MDX files. I followed the tutorial to create programmatically pages (here). I can see my files in GraphQL and in the list displayed in the blog index, but when I click on the link to open the page I have a 404 error. How can I fix this?

Steps to reproduce

I made a repo here: https://github.com/JulSeb42/gatsby-mdx. Just click on the link of any post.

Expected result

Blog post opening

Actual result

404 error

Environment

System:
OS: macOS 11.0.1
CPU: (4) x64 Intel(R) Core(TM) i7-4578U CPU @ 3.00GHz
Shell: 5.8 - /bin/zsh
Binaries:
Node: 14.15.0 - /usr/local/bin/node
npm: 6.14.8 - /usr/local/bin/npm
Languages:
Python: 2.7.16 - /usr/bin/python
Browsers:
Chrome: 86.0.4240.198
Firefox: 82.0.3
Safari: 14.0.1
npmPackages:
gatsby: ^2.25.3 => 2.25.3
gatsby-plugin-mdx: ^1.4.0 => 1.4.0
gatsby-source-filesystem: ^2.5.0 => 2.5.0
npmGlobalPackages:
gatsby-cli: 2.13.0