well the update finally came out on the evening of 25th May, and the fix for this isn't working properly as far as i'm concerned.
If you build a new machine, update it, then add to the domain, when it receives the logon/lock wallpaper gpo it applies successfully, so that part is fixed.
however if you update an existing domain joined machine, the update installs successfully and afaik fixes a few bugs (scroll bar on logon being one of mine), but the wallpaper is still not fixed.
I'm not rebuilding 90 machines that already have this policy
guess we'll have to wait another month for them to fix it properly, this issue and Edge browser being a totally unresponsive pain, are the only things holding the rest of my deployment back.