This has been identified as a known issue with IOS12 (AAPP-6478 in the KB here), but I wanted to share a workaround for devices where the wallpaper doesn't set properly - Change Organization Group on the device, but just keep the existing OG and hit Save. It repushes the Managed Settings config for that OG to that device, which will attempt to set the wallpaper again. Since the issue is intermittent this usually works (I did have to do it twice one time though). We are running a SaaS environment, so 1811 here.
Getting the same issue here on 1902, has this been fixed in newer AirWatch versions?