Hey @Kenji_Bailly — sorry about this, I just verified the same behaviour. We block json files in themes to prevent things like package.json being served. This should probably only apply in the root theme directory, but at the moment it looks like it applies to all directories - which is a bug.
I’ve passed this along to the team to track and improve in future. Unfortunately at the moment if you want to access a JSON file you’d need to host it outside Ghost, perhaps using something like a Cloudflare Worker