Parallax scroll not working

I’m trying to recreate a very simple parallax animation for my website. I want to increase the scroll speed of the main element of my hero section so it can end up behind the layer (the city image in this case). The problem is that as soon as I set the starting position to zero pixels, the selected element somehow disappear. Anyone can help me with that?

Here is my public share link:
(how to access public share link)