The same happens to me: the thumbnail of the active picture is not scrolled into the windows automatically.
To scroll the thumbnails you need to do an horizontal scroll.
Here you can verify it:
http://festivaldemenga.webflow.io/edizioni-precedenti
Hope someone can help, thanks in advance
Note:
The thumbs horizontal scrollbar is hidden below the window bottom, if the user scrolls down he can use it, but then the lightbox closes.