"anonymous" user connected to wireless

I'm having a strange problem, for one specific user, wireless system dashboard and radius accounting db show connected username as "anonymous". User credentials are stored in database and Calling-Station-Id is used to prevent unauthorized devices from connecting. FreeRADIUS authentication logs are not enabled. On DB, user "anonymous" do not exist. The wireless system is configured to authenticate only with FreeRADIUS server. Changing Calling-Station-Id on radius db prevents this system from connecting. I'm wondering how this is possible and where to check what is causing this. Thanks,