Code

<?php
add_filter('rwmb_meta_boxes', 'qrsi_select_advanced');

function qrsi_select_advanced($meta_boxes)
{
    $prefix = '';

    $post_id = null;
    if (isset($_GET['post'])) {
        $post_id = intval($_GET['post']);
    } elseif (isset($_POST['post_ID'])) {
        $post_id = intval($_POST['post_ID']);
    }

    $course_args = array(
        'post_type' => 'course',
        'relationship' => array(
            'id'   => 'event-to-course-relationship',
            'from' => $post_id,
        ),
    );

    $course_query = new WP_Query($course_args);

    if ($course_query->have_posts()) {

        while ($course_query->have_posts()) {

            $course_query->the_post();

            $course_post_id = get_the_ID();
        }
    }

    $meta_boxes[] = [
        'title'      => __('QRSI2', 'your-text-domain'),
        'id'         => 'qrsi2',
        'post_types' => ['events'],
        'include'    => [
            'relation'       => 'OR',
            'event-category' => [15],
        ],


        'fields' => [
            [
                'name'       => __('Schedule Block 1', 'your-text-domain'),
                'id'         => $prefix . 'schedule_block_1',
                'type'       => 'group',
                'clone'      => true,
                'sort_clone' => true,
                'fields'     => array(
                    array(
                        'name' => __('Course Name', 'your-text-domain'),
                        'id'   => $prefix . 'course_name',
                        'type' => 'post',
                        'post_type' => 'course',
                        'field_type' => 'select_advanced',
                        'columns' => 6,
                        'placeholder' => __('Select an Item', 'your-text-domain'),
                        'query_args' => array(
                            'relationship' => array(
                                'id'   => 'event-to-course-relationship',
                                'from' => $post_id,
                            ),
                            'post_status' => 'publish',
                            'posts_per_page' => -1,
                            'order' => 'ASC',
                            'orderby' => 'name',
                        )
                    ),

                    array(
                        'name' => __('Scholar Name', 'your-text-domain'),
                        'id'   => $prefix . 'scholar_name',
                        'type' => 'post',
                        'post_type' => 'scholar',
                        'field_type' => 'select_advanced',
                        'columns' => 6,
                        'placeholder' => __('Select an Item', 'your-text-domain'),
                        'query_args' => array(
                            'relationship' => array(
                            'id'   => 'from-course-to-scholar',
                            'from' => $course_post_id,
                        ),
                            'post_status' => 'publish',
                            'posts_per_page' => -1,
                            'order' => 'ASC',
                            'orderby' => 'name',
                        )
                    )
                )
            ],
        ],

    ];

    return $meta_boxes;
}