How can I underline whitespaces in VS Code using textMateRules?

I am using VS Code and would like to have markdown headings displayed underlined in the source code. I have added the following configuration:

"editor.tokenColorCustomizations": {
    "textMateRules": [
        {
            "scope": [
                "markup.heading.markdown",
            ],
            "settings": {
                "foreground": "#C0C3CA",
                "fontStyle": "underline",
            }
        },
    ],
}

This basically works. The spaces in between the words however are not underlined, see this screenshot:

Is there a possibility to have these underlined as well?

According to the scope inspector the spaces also have the markup.heading.markdown scope, just like the words. So I don't see why they do not get underlined.

Any ideas?