huh, yeah that is weird. are you just setting the background-image with css (no js when the page loads or anything fancy like that)? are you using your own style or modifying one of the templates?
can you set the background-image to something like background-image: linear-gradient(#fff, #000); ? if that doesn't produce a change, i suspect the rule is being invalidated by a css rule with higher specificity somewhere. if it does work, then maybe the image isn't being transferred over during the build - can you check if the bg image appears in the _site output folder?