I worked w/ @thewonglv from support earlier on this issue and what we produced properly works across all devices. Check out the current landing page site of https://webflow.com/website/mdftl (mdftl.webflow.io) to see how we accomplished this in lieu of the preview bug (keeping this site public for now at the time of this writing).
In http://mobile-fixed-background.webflow.io/, I see that the desired bkg image effect that we Webflow users are in need of when viewing from a mobile browser (Safari on an iPhone 5 in portrait or landscape mode in this case) is not produced. As follows:
When viewing http://mobile-fixed-background.webflow.io/ on an iPhone 5:
In portrait mode, the bkg image is un-proportionately skewed, as the image does not scale it to fill the bkg, plus the bkg image scrolls some upon scrolling with a bottom margin at times. In landscape mode, the same thing occurs, though the image is not skewed.
So the main issue here is that the Webflow preview shows one thing for tablet & mobile for the bkg image (and when we see that, we all usually agree with wanting that), but then that is different from what the actual browsers reveal, thus we cannot use the Webflow tools to properly design & utilize CSS to produce the desired bkg image effect - since we cannot properly preview it as we work.
Glad you guys are working through this bug and hopefully a permanent fix is released sooner than later.
Dylan // Digital Liberation