help with query with operator “EXISTS”

$example = new WP_Query( array(

? ? 'post_type' ? ? ? ?=> 'procedure',

? ? 'posts_per_page' ? => -1,

? ? 'post_status' ? ? ?=> array( 'publish' ),

? ? 'orderby' ? ? ? ? ?=> 'procedure_category',

? ? 'tax_query' => array(

? ? ? ? array(

? ? ? ? ? ? 'taxonomy' => 'procedure_category',

? ? ? ? ? ? 'operator' => 'EXISTS',

? ? ? ? )

? ? )

) );


in mu-plugins
function core_order_by_taxonomy_get_taxonomy( $wp_query ) {
    
    if ( !isset( $wp_query->query_vars['orderby'] ) )
        return false;

    $orderby = explode( ' ', (string)$wp_query->query_vars['orderby'] );
    foreach( get_taxonomies() as $tax ) {
        if ( in_array( $tax, $orderby ) )
            return $tax;
    }

    return false;

}
add_filter( 'posts_clauses', 'core_order_by_taxonomy', PHP_INT_MAX, 2);
function core_order_by_taxonomy( $clauses, $wp_query ) {
    global $wpdb;

    $taxonomy = core_order_by_taxonomy_get_taxonomy( $wp_query );

    if ( !$taxonomy )
        return $clauses;

    $clauses['fields'] .= ', CAST(term_order_meta.meta_value AS UNSIGNED) AS term_order_int';

    $clauses['join'] .= <<<SQL

LEFT OUTER JOIN {$wpdb->term_relationships} AS term_order_rel ON {$wpdb->posts}.ID = term_order_rel.object_id
LEFT OUTER JOIN {$wpdb->term_taxonomy} AS term_order_tax ON term_order_tax.term_id = term_order_rel.term_taxonomy_id
LEFT OUTER JOIN {$wpdb->terms} AS term_order_terms ON term_order_terms.term_id = term_order_tax.term_id
LEFT OUTER JOIN {$wpdb->termmeta} AS term_order_meta ON (term_order_meta.term_id = term_order_terms.term_id AND term_order_meta.meta_key = '_order_{$taxonomy}' )
SQL;
    $clauses['where'] .= " AND taxonomy = '{$taxonomy}'";
    $clauses['groupby'] = "term_order_rel.object_id";
    $clauses['orderby'] = trim( "COALESCE(term_order_int, ~0) ASC, " . $clauses['orderby'], ',');

    return $clauses;
}

add_filter( 'the_posts', 'core_order_by_taxonomy_final', 10, 2 );
function core_order_by_taxonomy_final( $posts, $wp_query ) {

    $taxonomy = core_order_by_taxonomy_get_taxonomy( $wp_query );

    if ( !$taxonomy )
        return $posts;

    $order_posts = array();
    foreach( $posts as $post ) {

        $terms = get_the_terms( $post, $taxonomy );

        foreach( $terms as $term ) {
            $order = get_post_meta( $post->ID, '_order_' . $term->slug, true );
            if ( empty( $order ) )
                $order = PHP_INT_MAX;

            $order_posts[ $term->slug ][] = array( 'order' => $order, 'post' => $post );
        }
    }  

    $posts = array();
    foreach( $order_posts as $term => $val ) {
        usort( $val, fn($a, $b) => $a['order'] <=> $b['order'] );        
        $order_posts[ $term ] = $val;
    }

    foreach( $order_posts as $term => $val ) {
        foreach( $val as $p ) {
            $posts[] = $p['post'];
        }
    }

    return $posts;
}