Same external URL

Hello,
My client uses a third party booking system. Their website has a button to launch the third-party URL. I would also like text links to point to the same third-party URL. However, this leaves multiple places throughout the website pointing to the same URL.
So the URL only needs to be changed in one place, I created a menu item of type System Links -> URL and 'picked' this menu item for both the button and text links through Gridbox.
I expected, if the URL of the menu item changed, all the Gridbox links would also change - but they do not. (please see images).

Why don't all the Gridbox links change when the menu URL is updated?

Thanks
Marc