First, there doesn't exist any feedback about that this KB will cause email missing. It isn't an issue that caused by this Security Update,
Use command below to check whether there exist emails in this mailbox Inbox and Sent Items:
Get-MailboxFolderStatistics User1 | ft Identity,FolderAndSubfolderSize
If emails exist in this mailbox, you can try to check from OWA or create a new profile. You can also use New-MailboxExportRequest to export emails from this mailbox.
If there real doesn't exist emails in this mailbox inbox, try to check whether there exist emails in "Deletions" or "Purges", if there exist emails in those folders, it means those email was delete by mistake. You can follow this picture below to recover those emails:
If there doesn't exist emails in those folders, you will cannot find and recover those emails.
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