Relationships + FE Submission: Trying to create relationship programmatically

I have 2 CPTs: Project and Update, and I need their relationship to be created programmatically at the time that Update posts are created with a front-end form on Project posts:

1. Project posts are displayed by a dedicated CPT-specific single template: single-project.php in the theme root.
2. Update posts are never displayed anywhere other than on the Project posts to which they are related (no single display, no archive display):
   • In register_post_type: 'has_archive' => false
   • In register_post_type: 'publicly_queryable' => false
3. Every Project post is expected to be related to one or more Update posts *AND* every Update post is only ever related to the single Project post on which it was created:
   • MB Relationships installed

   • 
     add_action( 'mb_relationships_init', function() {
         MB_Relationships_API::register( [
             'id'   => 'projects_to_updates',
             'from' => [
                 'admin_column'     =>  true,
                 'empty_message'    => 'Empty "from"...',
                 'object_type'      => 'post',
                 'post_type'        => 'project',
             ],
             'to'   => [
                 'admin_column'     => 'after title',
                 'empty_message'    => 'Empty "to"...',
                 'object_type'      => 'post',
                 'post_type'        => 'update',
             ],
         ]);
     });
     

4. Existing Update posts that have been manually related to Project posts (instead of programmatically) are listed on the Project post with:

   
   $related_posts = new WP_Query([
       'nopaging'     => true,
       'order'        => 'ASC',
       'orderby'      => 'date',
       'relationship' => [
           'from'     => get_queried_object_id(),
           'id'       => 'projects_to_updates',
       ],
   ]);
   
   if ( $related_posts->have_posts() ) {
   
       while ( $related_posts->have_posts() ) {
   
           $related_posts->the_post();
   
           ?>
   
               <div>
                   <h3><?php echo get_the_title(); ?></h3>
                   <div><?php echo get_the_date(); ?></div>
                   <div><?php echo get_the_author(); ?></div>
                   <div><?php echo rwmb_meta( 'update_post', $table_args_for_updates_table ); ?></div>
               </div>
   
           <?php
   
       }
   
       wp_reset_postdata();
   
   }
   

5. New Update posts are created on the front end with a form that appears on every Project post:
   • MB Frontend Submission installed
   • In single-project.php: echo do_shortcode( '[mb_frontend_form ajax="true" id="update-fields-group" post_fields="title,update_post"]' );

   • 
     add_action( 'rwmb_frontend_after_process', function( $config, $post_id ) {
         MB_Relationships_API::add( get_queried_object_id(), $post_id, 'projects_to_updates', $order_from = 1, $order_to = 1 );
     }, 10, 2 );
     

The problem is that while Update posts are indeed created, there is never a relationship created between these new Update posts and the Project post on which they were created. If I relate them manually, they display on the Project post as expected.

So, I know I'm almost there with this first round of requirements, but even after reviewing multiple threads here, I cannot get the relationship to be created at Update post creation.