mu-lugins do not load

[jeremyfrank]

I'm sure I'm missing one small step, but using the following config, mu-plugins appear to not be loaded completely in the WP admin.

composer.json

{
    ...
    "repositories": [
        {
            "type": "composer",
            "url": "https://wpackagist.org"
        }
    ],
    "require": {
        "johnpbloch/wordpress": "5.2",
        "lkwdwrd/wp-muplugin-loader": "~1.0.0",
        "wpackagist-plugin/advanced-custom-fields": "5.8",
        "wpackagist-plugin/contact-form-7": "5.1.3",
        "wpackagist-plugin/duracelltomi-google-tag-manager": "1.9.2",
        "wpackagist-plugin/safe-svg": "1.9.4",
        "wpackagist-plugin/timber-library": "1.9.4",
        "wpackagist-plugin/wordpress-seo": "11.2.1"
    },
    "extra": {
        "force-mu": [
            "advanced-custom-fields",
            "contact-form-7",
            "duracelltomi-google-tag-manager",
            "safe-svg",
            "timber-library",
            "wordpress-seo"
        ],
        "installer-paths": {
            "wp-content/mu-plugins/{$name}": [
                "type:wordpress-muplugin"
            ]
        },
        "wordpress-install-dir": "wp"
    }
}

Directory (after running composer update)

Contents of generated mu-require.php file

<?php
require_once __DIR__ . '/../../vendor/lkwdwrd/wp-muplugin-loader/src/lkwdwrd/mu-loader.php';

WP Admin

The timber error is coming from a custom theme which uses timber. The WP menu should have "Custom Fields" and "SEO."