UPDATE:
Microsoft has reported that a fix has been deployed across all networks which should mitigate the issues for Outlook users.  Users are recommended to restart their Outlook client for the changes to take effect.
ORIGINAL POST:
Microsoft Outlook is currently experiencing an issue with email visibility in Outlook. OWA does not appear to be affected at this time.At the time of the original post, Microsoft has reported the incident in the Service Health page of the Microsoft 365 Admin Center and in their Microsoft 365 Status Twitter feed (@MSFT365Status).  Microsoft has not provided any details about the incident the issue on their health status page, status.office365.com. These issues are solely on the Microsoft systems and only affect users of the Microsoft 365 email service. We will be happy to record and report any issues you might be experiencing.