We have been experiencing a lot of issues recently with Shared Mailboxes in Outlook 2016 not staying up to date. This can be resolved by configuring 2 Group Policy settings.
These settings still allow the primary user mailbox to be cached to improve performance, but the Shared Mailboxes work Online so do not cache and remain up to date all the time.
The 2 settings you need to configure are:
User Configuration/Policies/Administrative Templates/Microsoft Outlook 2016/Account Settings/Exchange/Cached Exchange Mode
Download shared non-mail folders – Disabled
User Configuration/Policies/Administrative Templates/Microsoft Outlook 2016/Outlook Options/Delegates
Disable shared mail folder caching – Enabled
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