To resolve this issue, please do the following:
- Go to the Microsoft 365 admin center
- Go to Settings | Org settings | Reports
- Clear the "Display concealed user, group, and site names in all reports" option.
NOTE: When you change the setting to turn off user-level data anonymization, the change is not in effect until On Demand synchronization with the tenant has occurred. Synchronization can take up to six hours.