It's hard to tell what FortiAuthenticator is having an issue with just based on the difference in username format. It could be that it's not matching the correct RADIUS policy, or failing at some stage after matching a RADIUS policy.
You can check the RADIUS debug on FortiAuthenticator via GUI: https://<FortiAuthenticator>/debug/radius -> you should see RADIUS requests (and what FortiAuthenticator does with them) there, and it should provide additional information why login with CIRCUS\firstname.lastname fails, but login with firstname.lastname is successful
+++ Divide by Cucumber Error. Please Reinstall Universe and Reboot +++