After a little puzzling with MS Outlook 2016 Synchronization settings GPO I thought I’d share this small bug in the GUI (at least for our environment 2k8r2 ADs):

If you’re editing Outlook 2016 cached exchange mode duration:

\User Configuration\Policies\Administrative Templates\Microsoft Outlook 2016\Account Settings\Exchange\Cached Exchange Mode\Cached Exchange Mode Sync Settings

Then switching the option to “All” and re-opening the policy in the GUI will display it reverting to “Three Days”, this is a bug!

My solution was simply changing the setting to “All” and not returning to the GUI control of the policy, if I then exited the policy and returned to the Group Policy Management GUI, inspecting the policy’s [Settings] I found the “All” setting was retained behind the scenes.