Hi,
The problem comes from the JavaScript code of your template handling the fixing of the navigation bar at the top when you scroll down the page.
The page height on the product page of your link is the exact size where that problem would happen.
Add some height to the product page, and you'll see that the problem won't happen.
You can do that for example by adding such CSS:
.hikashop_product_page{ height : 700px; }
So adding extra vertical space to the product page is a solution, but a more elegant solution would be to fix the scrolling javascript code of the template. You might want to contact your template provider about that.