I applied the latest hotfix (15) and tried with various combinations of users/roles and cannot find a pattern how it works. Some roles see only users from the same role and some roles see all. It may have something to do with being editor. Not sure what is happening, but we cannot use this web part until works without surprises. I definitely do not see any online users as global admin in CMSDesk.
If I may suggest I would add new settings to the web part: Display users from role, Include editors, Include global admins

Thanks,
Vlado

I increased the setting Site Manager->Settings->Security & Membership->Update on-line users (minutes) from the default 1 to 3 and it seems to be working OK.

Regards,
Vlado