Hi there, thanks, this appears to be a background-attachment scroll issue similar to
That is an open issue at the moment with some browsers/os that is changing the background-attachment from Fixed to Scroll when the page is rendered.
Here btw is how the fixed background image looks for me on Chrome:
https://cl.ly/0L023t1m2E3t/Screen%20Recording%202018-01-05%20at%2008.25%20AM.mov
In the meantime, I would check the workaround provided in the post link above, which is to add a fixed DIV to the page, with a z-index of -1 and a page full cover.
In nearly all cases I have found, applying the fixed position to the html tag DIV instead of to the background-attachment style attribute will work better on mobiles/other browsers.