Shared mailbox is added as a second exchange account in cached mode in outlook 2016. Microsoft does not recommend to add shared mailbox as a separate account in outlook. This is non-supportive scenario for us. However we can provide best effort support in this case. We tried following steps to resolve the issue,

• We were not able to search items in current folder but Subfolders; All Outlook Items; Current Mailbox; All Mailboxes in cache mode.
• We were able to search items in current folder in online mode.
• Unchecking download shared folder from account settings did not make any difference.
• Issue is reproducible in safe mode, different machines, different networks, new user profile and other user who has access to this shared folder.
• If we add normal user account to outlook, search works fine.
• Outlook and windows are up to date.
• Rebuilding indexing did not fix the issue.
• Unchecking search option, “Improve search speed by limiting the number of results shown” did not make any difference.
• Search works fine in current folder after auto mapping of shared mailbox in outlook by running PowerShell commandlet,

Please find below search scopes in Outlook,

Reference: https://blogs.technet.microsoft.com/outlooking/2014/04/28/understanding-search-scopes-in-microsoft-outlook/

• As the size of OST is around 20 GB so you may face performance issues in outlook while accessing secondary mail.

Reference:

https://support.microsoft.com/en-us/help/3115602/performance-problems-when-you-try-to-access-folders-in-a-secondary-mai

https://support.office.com/en-us/article/Reduce-the-size-of-Outlook-Data-Files-pst-and-ost-E4C6A4F1-D39C-47DC-A4FA-ABE96DC8C7EF?ui=en-US&rs=en-US&ad=US

https://support.microsoft.com/en-us/help/2759052/you-may-experience-application-pauses-if-you-have-a-large-outlook-data

http://go.microsoft.com/fwlink/?LinkID=267344

https://support.office.com/en-us/article/Fixes-or-workarounds-for-recent-issues-in-Outlook-for-Windows-ecf61305-f84f-4e13-bb73-95a214ac1230