@artman2 just to reiterate what @dram said, the issue can be resolved using a div with the image background set to “fixed”. Note: this workaround did not require the use of custom header code.
I found this forum post from 2016 explaining the issue:
A work around which you can try is set a Fixed element as the background, try the following:
** Add in a div, call it “Fixed background” (or whatever you wish)*
** Set the div to Position: Fixed*
** Width: 100%, Height: 100vh (type in 100vh so that it takes up the full viewport)*
** Adjust the z-index of this element to sit behind all other elements"*
Source: Trouble with fixed background images not remaining fixed on touch screen devices