Metaboxes not saving values

On the editor screens for two CPTs and for user profile, I see the meta screens to make connections. However, the connections aren’t saving on any of those screens.

There aren’t any javascript errors showing in the console.

I’m running the following plugins:

Meta Box
MB User Meta
MB Relationships

I’ve registered two connections:

add_action( 'mb_relationships_init', function () {

    MB_Relationships_API::register( array(
        'id'   => 'programs_to_updates',
        'to' => 'projects',
        'from'   => 'updates',
    ) );

    MB_Relationships_API::register( array(
        'id'   => 'users_to_posts',
        'to' => array(
            'object_type' => 'user',
            'meta_box'    => array(
                'title'       => 'Program access',
                'field_title' => 'Select projects',
            ),
        ),
        'from'   => array(
            'object_type' => 'post',
            'post_type'   => 'projects',
            'meta_box'    => array(
                'title'         => 'Clients with access',
                'context'       => 'side',
                'empty_message' => 'No users',
            ),
        ),
    ) );
} );