Outlook 2019 Missing Email until turn on Cache Mode

Hello;

One of my user reported missing emails, the email she sent was not in Outbox, not in Sent Item, I checked the Exchange 2013 mail flow report which did no show there.  When I diagnosed her problem, I found that the Outlook cache mode was turned off.  After I turned on the Cache Mode, the email reappear on Outbox and sent immediately.

If someone facing the same issue?  Why I turned on the Cache Mode, the email reappear in Outbox again.  Subject to my assumption, I think that the email was composed and it was being cached somewhere in user profile, after re-enabled the Cache Mode on Outlook 2019, it triggers the index or rescan, so the email reappear in Outbox.

Can anyone give me some hints.

thanks!

KW - CNE,MCSE,VCP5