Hi,
We found and fixed the issue. The affected account has Joomla's "Increased Contrast" accessibility option enabled, which applies a CSS filter: contrast(115%) to the body element. Per the CSS specification, this breaks position: fixed positioning for all descendant elements, causing HikaShop's popups to be positioned relative to the page body instead of the viewport.
This is why the popup only fails when the page is scrolled down. When you're at the top of the page, there's no difference. But once you scroll, the popup ends up above the visible area.
We've implemented a fix in HikaShop that detects the CSS filter and compensates the popup positioning. You can download again the install package on our website and install it on yours to get the patch. You'll have to do a hard refresh on the backend to see the change.