relationship doesn't save?

I have defined a reciprocal relationship:

add_action( 'mb_relationships_init', 'your_prefix_function_name' );

function your_prefix_function_name() {
    MB_Relationships_API::register( [
        'id'         => 'registration-partner',
        'reciprocal' => true,
        'from'       => [
            'object_type'   => 'post',
            'post_type'     => 'registration',
            'empty_message' => 'no partner',
            'admin_column'  => [
                'position' => 'after title',
                'title'    => 'Partner',
                'link'     => 'edit',
            ],
            'meta_box'      => [
                'title'   => 'Partner',
                'context' => 'normal',
            ],
            'field'         => [
                'name'              => 'Partner',
                'label_description' => '= connected registration',
                'max_clone'         => '1',
            ],
        ],
        'to'         => [
            'object_type'   => 'post',
            'post_type'     => 'registration',
            'empty_message' => 'no partner',
            'field'         => [
                'name'              => 'Partner',
                'label_description' => '= connected registration',
                'max_clone'         => '1',
            ],
        ],
    ] );
}

and inside a function hooked into save_post_registration I am doing this

                $query = new WP_Query($args);

                if ($query->have_posts()) {
                    $partner_id = $query->posts[0]->ID;

                    error_log("current post: " . $post_id);
                    error_log("current partner: " . $partner_id);

                    if (!MB_Relationships_API::has($partner_id, $post_id, 'registration-partner')) {
                        $relationship = MB_Relationships_API::add($post_id, $partner_id, 'registration-partner', 1, 1);
                        error_log($relationship);
                        $relationship = MB_Relationships_API::add($partner_id, $post_id,  'registration-partner', 1, 1);
                        error_log($relationship);
                    }

                    #MB_Relationships_API::add($post_id, $partner_id, 'registration-partner', 1, 1);
                    # TODO - generalize ID?
                    if (1) {
                        rwmb_set_meta($post_id, $this->plugin_prefix . 'partner_registered', 1);
                    }
                }

But the relationship is never saved.

the output of the error.log is:

[06-Oct-2022 17:12:00 Europe/Berlin] current post: 68
[06-Oct-2022 17:12:00 Europe/Berlin] current partner: 143
[06-Oct-2022 17:12:00 Europe/Berlin] 1

which indicates a true being returned from MB_Relationships_API::add