Applying Local Computer policy not "sticking"

Bit of a weird one this, testing some policies with ZCM and Windows 7 Enterprise SP1 x86...

- set a local policy to always use custom logon background
- create a bundle to copy wallpaper files
- policy applies on first boot
- policy appears in rsop.msc correctly
- log-off \ reboot after and the logon background resets to Windows default
- sometimes when logging off I see the background flick from the custom one to Windows one
- checking the registry suggests the setting now isn't applied (registry key shows 0 instead of 1)

Thoughts?