Background Image always scrolls on iOS

[migwellian]

Hi there, I've found that the "Background Image" feature does not work properly on iOS: the image scrolls with the page even when configured not to.

I reproduced the issue with a clean install of the latest Underscores theme, then setting a Background Image with Preset = "Fill Screen" or "Fit to Screen" or "Custom". While it behaves as expected on my laptop (using Firefox or Chrome), it does not behave correctly on mobile devices (using Safari or Chrome).

I have reproduced the issue on three different client machines: an iPhone 12 mini with iOS 15.5, an iPhone 8 with iOS 14.8.1, and an iPad Air with iOS 12.5.5.

Thanks!