When looking the SharePoint Site Usage detail reports (https://learn.microsoft.com/en-us/graph/api/reportroot-getsharepointsiteusagedetail?view=graph-rest-1.0&tabs=http) the Site URL parameter is always empty.
This is true for both calling the report graph api endpoint (https://graph.microsoft.com/v1.0/reports/getSharePointSiteUsageDetail(period='D7')) and when browsing the report in the admin portal (https://admin.microsoft.com/#/reportsUsage/SharePointSiteUsageV1)
I have checked several different tenants and the behaviour is the same for all. I've also tried with switching the deanonymizing on/off with no effect either.
Ok, this looks like it is a known issue that's been happening for a while.
There is currently an open service health advisory at
https://admin.microsoft.com/#/servicehealth/:/alerts/SP676147
Title: Admins may see incomplete usage reports for SharePoint Online and OneDrive for Business
User impact: Admins may see incomplete usage reports for SharePoint Online and OneDrive for Business.
More info: SharePoint and OneDrive URLs may not be displayed in some usage reports. The following report types may be impacted:
As a workaround, admins with appropriate access can use the SharePoint REST API or Microsoft Graph API to retrieve information about site IDs referenced in the affected reports.