I've had a look at my report useage metrics today and I've been getting lots of views with a blank username since 29th September.
Not only that but the volume of these views is unusually high. Usually I wouldn't expect to have more than 10 views in a day for one report but on one of these days there are over 100,000 views with this blank user.
Anyone got any ideas what is causing this?
Based on my test, I cannot reproduce your issue here.
You can create a ticket for help here.
Were you ever able to find a solution to this? I am experiencing the same issue on some highly sensitive information and would like to know who is looking at the report/fix it so that it does not happen again.