ACF Template Partials in a mu-plugin Class

content-page.php

<?php
/* Wherever you place this code, the ACF partials for the post will output. I tend to put it after the default content. */
  $tmpart = new <name of class>();
  $tmpart->tmpltPartials();
?>

theme-class.php

<?php

/**
 * @wordpress-plugin
 * Plugin Name:       <name of client> Main
 * Plugin URI:        <URI>
 * Description:       Functions required by <name of theme> theme
 * Version:           1.0.0
 * Author:            <author name>
 * Author URI:        <author URI>
 * License:           GPL-2.0+
 * License URI:       http://www.gnu.org/licenses/gpl-2.0.txt
 * Text Domain:       <same as for the theme>
 * Domain Path:       /languages
 */

// If this file is called directly, abort.
if (!defined('WPINC')) {
    die;
}

add_action('init', ['<name of class>', 'init']);

register_activation_hook(__FILE__, ['<name of class>', 'activatePlugin']);

class <name of class>
{
    /**
     * Creates a new instance.
     *
     * @wp-hook init
     *
     * @see    __construct()
     */
    public static function init()
    {
        new self();

        // Hide the WordPress generator meta
        add_filter('the_generator', function () {
            return '';
        });
    }

    /**
     * <name of class> constructor.
     */
    public function __construct()
    {
        add_filter('body_class', [$this, 'bodyClasses']);
        add_filter('post_class', [$this, 'removeHEntry']);

        // Enable SVG upload and display
        add_filter('upload_mimes', [$this, 'enableSVGUpload']);
        add_action('admin_head', [$this, 'enableSVGDisplay']);

        // Shortcodes
        add_shortcode('shortcodetest', [$this, 'shortcodeTest']);

    }

    /**
     * Adds custom classes to the array of body classes.
     *
     * @param array $classes Classes for the body element.
     *
     * @return array
     */
    public function bodyClasses($classes)
    {
        global $post;

        // Adds a class of group-blog to blogs with more than 1 published author.
        if (is_multi_author()) {
            $classes[] = 'group-blog';
        }

        // Adds a class of hfeed to non-singular pages.
        if (!is_singular()) {
            $classes[] = 'hfeed';
        }

        if (isset($post)) {
            $classes[] = $post->post_type.'-'.$post->post_name;
        }

        return $classes;
    }

    /**
     * Remove hentry from pages.
     *
     * WordPress adds hentry to posts and pages, but hentry should only be for posts.
     *
     * @param $classes
     *
     * @return array
     *
     * @link https://core.trac.wordpress.org/ticket/28482
     */
    public function removeHEntry($classes)
    {
        if (is_page()) {
            $classes = array_diff($classes, ['hentry']);
        }

        return $classes;
    }

    /**
     * Enables SVG uploads in the Media Library.
     *
     * @param $mimes
     *
     * @return mixed
     */
    public function enableSVGUpload($mimes)
    {
        $mimes['svg'] = 'image/svg+xml';

        return $mimes;
    }

    /**
     * Enables proper SVG display in the Media Library.
     */
    public function enableSVGDisplay()
    {
        echo '<style>kp.media-icon img[src$=".svg"], img[src$=".svg"].attachment-post-thumbnail { width: 100% !important; height: auto !important; }</style>';
    }

    /**
     * Shortcode: Button.
     *
     * @param $atts
     * @param null $content
     *
     * @return string
     */
    public function shortcodeTest($atts, $content = null)
    {
        extract(shortcode_atts(
                [
                    'foo' => '',
                    'bar' => '',
                ], $atts)
        );
        $results = $foo . " > " . $barr;
        return $results;
    }

    /**
     * Partials.
     * Original Source: https://selfteach.me/acf-flexible-content-modular-wordpress-themes/
     * When you define a new content block in ACF, you'll need to create a partial file called partial-*.php.
     * The * being the slug of the content block. It must be exactly the same, otherwise it will not load.
     */
    public function tmpltPartials()
    {
        // Create an array to hold the content blocks. This keeps them in order.
        $results = [];

        if (have_rows('layout')) {
            while (have_rows('layout')) {
                the_row();

                if (get_row_layout() === 'template_picker') {
                    $file_name = basename(get_sub_field('template'), '.php');
                    $template = substr($file_name, strpos($file_name, '-') + 1);

                    ob_start();
                    get_template_part('template-parts/partial', $template);
                    $results[] = ob_get_clean();
                } else {
                    ob_start();
                    get_template_part('template-parts/partial', get_row_layout());
                    $results[] = ob_get_clean();
                }
            }
        }

        foreach ($results as $result) {
            echo $result;
        }
    }
}