Field Values Not Saving (Sometimes)

I've tried increasing but I already had it set pretty high. I was on 100000 before and bumped it up to 1000000 with no change in behaviour.

I've made further tests and this is the result. All value updates made by the admin user through the form are working fine.

All value changes made by a standard user are failing. I've even reduced the form to it's minimum level as below with only one remaining field which still fails to save.

I wonder if there are settings against the user role which I need to check to ensure the values are saved to the database? I use Members from MemberPress for the sole reason of creating my custom users roles if there may be an issue there?

add_filter( 'rwmb_meta_boxes', 'test_cip_pet' );

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

    $meta_boxes[] = [
        'title'   => __( 'TEST - Pet Details', 'your-text-domain' ),
        'id'      => 'test_pet_details',
        'type'    => 'user',
        'include' => [
            'relation'         => 'AND',
            'user_role'        => ['um_client', 'administrator'],
            'edited_user_role' => ['um_client'],
        ],
        'fields'  => [
            [
                'id'     => $prefix . 'group_pet_details',
                'type'   => 'group',
                'fields' => [
                    [
                        'name'        => __( 'Specify details for each of your pets.', 'your-text-domain' ),
                        'id'          => $prefix . 'group_pet',
                        'type'        => 'group',
                        'collapsible' => true,
                        'group_title' => '{pet_name} - {pet_breed}',
                        'clone'       => true,
                        'max_clone'   => 20,
                        'add_button'  => __( 'Add another pet...', 'your-text-domain' ),
                        'fields'      => [
                            [
                                'name'     => __( 'Pet\'s Name', 'your-text-domain' ),
                                'id'       => $prefix . 'pet_name',
                                'type'     => 'text',
                                'required' => true,
                            ],
			],				
                    ],
                ],			
            ],
        ],
    ];

    return $meta_boxes;
}