`writeMany` is not overwriting existing values

[edalzell]

I'm trying to write to Statamic's cp.php config.

My code:

        ConfigWriter::writeMany(
            'statamic.cp',
            [
                'enabled' => env('CP_ENABLED', true),
                'route' => env('CP_ROUTE', 'cp'),
                'start_page' => 'collections/pages',
                'widgets' => [],
                'date_format' => 'Y-m-d',
                'pagination_size' => 100,
                'link_to_docs' => false,
                'support_url' => env('STATAMIC_SUPPORT_URL', 'https://www.transformstudios.com/contact'),
                'theme' => env('STATAMIC_THEME', 'simple'),
                'custom_cms_name' => env('STATAMIC_CUSTOM_CMS_NAME', env('APP_NAME', 'Simple')),
                'custom_logo_url' => [
                    'nav' => '/img/transform-studios-avatar-circle.svg',
                    'outside' => '/img/transform-studios-logo-handdrawn.svg',
                ],
                'custom_favicon_url' => '/img/favicon.ico',
                'custom_css_url' => '/css/cp.css',
                'support_url' => env('STATAMIC_SUPPORT_URL', 'https://www.transformstudios.com/contact?utm_campaign=contextual+support&utm_source=cp+support+link&utm_medium=text+link'),
            ]
        );

But the existing values in that file are not overwritten with the ones above.

It looks like the previewMany returns the existing values not the existing + merged ones.