When you use the Microsoft Outlook 2016 administrative template to configure the Cached Exchange Mode Sync Setting policy to "One week", "Two weeks", or "All", the Group Policy Management Console (GPMC) does not differentiate between these options, and it always shows "Three days" when you reopen the management console.
This only affects the way that the setting is displayed in GPMC. Actually, the Cached Exchange Mode Sync Setting policy works correctly as it's configured.
Note You can also configure the cache mode setting by using the SyncWindowSetting and the SyncWindowSettingDays registry keys. For more information, see Update lets administrators set additional default Sync Slider windows for new Exchange accounts in Outlook 2016.