Why aren't sub-settings recognised in VS Code? [duplicate]

Consider the following settings.json file.

{"[markdown]": {"editor": {"tabSize": 4,"insertSpaces": true,"detectIndentation": false        }    },"[json]": {"editor.tabSize": 4,"editor.insertSpaces": true,"editor.detectIndentation": false    }}

The json section is bright whereas the markdown section is dull - suggesting to me that the markdown configuration isn't recognised by VS Code.

If I move my mouse over the json section then VS Code shows a description of the setting. If I move my mouse over the markdown section then I am shown an "Unknown Configuration Setting" message, yet both the markdown and json settings work in VS Code.

If both 'layouts' shown above work then why doesn't VS Code correctly recognise both of them?