Office 365 premium Outlook sent items search not working on client computer

[JP_Leapers]

One of our staff's Outlook (Office 365 premium on local computer) sent items folder search not working , pop up message No matches found. when search Inbox or other folders, search is working, When log into his Office 365 cloud Outlook,  search sent items folder works fine. Tried to rebuild index and all other suggestions can found on google, the issue still there. even with a brand new computer and install Office Premium with the same account, the issue still there, if install with other users account, search is working on sent items folder.

[#2]

Hi,

Just wondering but did you enabled Offline caching (ost)? and if you did for how long?

[#3]

Yes, we did set one year

[#4]

We have 30+ premium users using offline Office365 Outlook in the same office, no one else have reported this issue before (someone had search not working on whole Outlook, we did index rebuild and fixed it), this staff only has this issue on Sent Items folder since two weeks ago. he is our HR chief and did send some encrypted emails, is this a cause?