There is a fairly well known issue related to the display of background images on mobile devices. The problem is related to the fact that modern browsers on mobile devices have an address bar that disappears when you start scrolling.This causes resizing of the viewing area of the site. Accordingly, the image to which you have given 100% of the area should expand. And it expands, i.e. constantly twitching because of the movements of the address bar.

Links to fix this problem:

> CSS part: CLICK
> JS part: CLICK