Parallax background image visible when scrolling to top of a page

I have created a parallax image as a background image, where I can scroll content on top of the image, while the image stays fixed. This works, except for that when I scroll and the page loads, it is shown as the background. Also when I get to the top of the page and to the bottom of the page, the whole page kind of “lifts” and I can see the fixed image underneath.

Here’s a video of the problem: Screen-Recording-2022-11-01...

Here is my public share link: