Yeah that happens on pretty much any site or theme that uses Parallax/Large background images. The Background Images are designed to only Scale so far using CSS (without being distorted). Once you have a lot of content that exceeds beyond the size or height of the image, then the image will stop while the content keeps going. That’s why you’ll see a background image with content that continues to go beyond the image.
What a lot of developers do in this situation is create a second image that only shows up when viewed on mobile devices. This image is usually edited to handle a larger height. I’ve seen some people change the image entirely, or remove it and have a background color show up when being viewed on mobile devices.
You should be able to do that with whatever Photo Editing software you’re using……assuming you’ve used one to create the images you have (if not, then we’ll have to come up with some alternatives)