Just had a tech run into this and wanted to warn people and get it fixed.
A tech had just added a tenant level admin but wanted to delete it so they could name it something else. They clicked on the box next to the name and clicked delete at the bottom - see picture.
The next popup only asks 'Are you sure you want to delete?' - see picture. He deleted the entire tenant!
It should read 'Are you want to delete the entire Tenant' and then pop 'are you sure - all tenant user information will be deleted and it is nonrecoverable'
After he showed me the process I stopped yelling at him b/c it is reasonable to assume you are only deleting the admin. There should be a lot more you have to do to delete a tenant.
Be warned & please fix!