This is not a bug. The css code "
background-attachment: fixed" simply does not work on most mobile devices, supposedly because it drains too much CPU (battery) on scrolling. See reference below, or you can also search Google.
http://caniuse.com/#feat=background-attachment
You can see the difference in the link below also, by comparing desktop vs mobile:
https://demo.photo.gallery/examples/plu ... tro-image/
On desktop, you will see the image remains fixed, while on mobile, the image will scroll with page. The fixed attachment should be considered a "progressive" enhancement. Having it fallback to "scroll" on mobile devices, is normally quite harmless to the effect itself.
We could implement a "hack" for this feature, but it's not worth the hassle, since ultimately it will achieve something mobile devices are attempting to avoid.