Z-index does not work on mobile and tablet

Hello,

I have a z-index on my sticky animation so that the circle goes over the first circle.
it works on pc, but not on tablet and mobile.

However, from designer mode in mobile format, it works. But when I test on my phone, on chrome, safari… it doesn’t work.

The desired effect :

Here is the read-only site :
https://preview.webflow.com/preview/point-median?utm_medium=preview_link&utm_source=designer&utm_content=point-median&preview=cb40260b1a11f10ebe435d0bd449b48e&workflow=preview


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