It searches the "Display Name "
During the ubbt 7 5.x series, the control panel often used the term "username" interchangably for "Login Name" and "Display Name". A lot of work was done for the 7.6.x series to clean up that confusion. What you are seeing is one of the "username" items that slipped by and did not get fixed.
In this case, "username" here means "Display Name."
Thanks for reporting this. At the time of reading your post, I updated the software to reflect this correction. Good find