Hello,
This is a thread that dates back to 2 months ago and does not seem to reach to a solution. I came across this thread yesterday due to the same problem I am having. So below link apparently solved my problem. I hope it does it for you too. Then again, I guess squarespace added parallax effect for background images for its updated 7.1 platform. FYI.
Here's the link: https://schwartz-edmisten.com/blog/how-to-add-parallax-to-squarespace-7-1