We have a corporate wallpaper GPO in place since the early Windows 7 era that copies a .bmp file to the user's %temp% and then uses %temp%\corp.bmp as the path. It works for everyone, 99 times out of a hundred. For the hundredth person, it works too, until it doesn't. Instead, they get a black background -- no corporate wallpaper, and as specified by the GPO they can't make any changes.
The .bmp file is accessible and properly permissioned. If we delete it and run gpupdate /force the file is restored. If we overwrite the registry setting, say with c:\temp\corp.bmp, that doesn't work either, and in any case as soon as the policy executes our customization is overwritten.
For one past user we went to the trouble of blocking the GPO. She got her background back. When we unblocked the GPO she went back to black.
In the past five years this has only ever happened to remote users who launch a desktop VPN client and only get the policy when it refreshes periodically.
A fix someone in the field discovered is that if you go to Ease of Use and disable and then re-enable the desktop background, the wallpaper will reappear. That has not worked for Mr. Hundredth.
This seemed to be a common problem for early Win 7 systems (though we never saw it) and most of the solutions refer to a Win 7 hotfix or else just make sure the .bmp is accessible (see above). The problem guy was on Win10.1803 and the problem persists after we upgraded him to Win10.1809.
Does anybody have advice for when the basics have been covered and it still don't work?