On 9/1/2021 Microsoft started storing usernames as FJEHSDFJDSH243424234234234 unless users disable it in their settings. See solution below. There may be a waiting period before the settings take effect (they may need to wait 1-2 days if there’s a delay before the setting actually takes effect):
"An Office 365 administrator must disable the setting "Display anonymous identifiers" under Settings->Reports."
Microsoft 365 reports show anonymous instead of actual user names - Office 365
Microsoft 365 reports show anonymous user names instead of the actual user names in the following reports:
Email Activity
Mailbox Activity
OneDrive files
SharePoint Activity
SharePoint Site Usage
Microsoft Teams Activity
Yammer Activity
Active users in Microsoft 365 Services and Apps
Groups Activity
To see these reports in the Microsoft 365 admin center, go to Home > Reports > Usage.
Cause
Starting September 1, 2021, we're hiding user information by default for all reports as part of our ongoing commitment to help companies support their local privacy laws. For more information, see Microsoft 365 Reports in the admin center and Privacy changes to Microsoft 365 Usage Analytics.
Resolution
To fix this issue, change the following setting in the Microsoft 365 admin center.
Note
You must be a Microsoft 365 global administrator.
- Go to the Microsoft 365 admin center.
- Go to Settings > Org Settings > Services.
- Select Reports.
- Clear Display concealed user, group, and site names in all reports, and then select Save."