"Make proxy settings per-machine (rather than per user)" Group Policy setting not applied until login as a local Administrator

We want to deploy to all our desktop the pac file to configure proxy. We have a Windows 2008 R2 server, and i've enabled the GPO "Make proxy settings per-machine (rather than per user)", and i've add a registry key AutoConfigURL in "HKLM\Software\Microsoft\Windows\Current Version\Internet Settings" with the pac file link.

I've tested on my pc, and all was configured without any problem. I've try to login to my computer with another user (without admin rights) and the automatic configuration proxy was compiled and not modificable. It's seems that all works.

But, our users are not local admin, so i've tried to deploy the GPO in a collegue computer. I've forced the update of GPO, checked on registry that all new keys are added, and i've reboot the pc. When i've check on IE settings, autoconfig URL was empty and grey. I'm disconnected from user and i've login to the pc with a local admin. With my surprise, the IE settings was compiled. When i'm come bac to the user profile the IE settings was compiled and not modificable.

The problem is: i've over 750 users in 3 countries, and i don't want grant them the local admin permissions. How can i configure proxy settings via GPO without login to every machine at least one time?