Enqueue styles in new site editor in WordPress 5.9

So apparently for the site editor, you need to add the styles a little bit differently. You need to use the add_editor_style function.

add_action('after_setup_theme', 'my_add_editor_styles_to_full_site_editing');

function my_add_editor_styles_to_full_site_editing()
{
    add_theme_support('editor-styles');
    add_editor_style('site-editor.css');
}

Becareful how you style your stylesheet

This function adds your styles inline wrapped in the .editor-styles-wrapper class.