The system does not default to prefixed-based random usernames. Someone must have configured this. Check the create_multi form under Customization > Forms & Views, and make sure random_username_method has an appropriate value. If you search the inline Help for random_username_method you should get sufficient detail. Here are some of the common options:
'nwa_digits_password' => NwaText(1122, 'Random digits'),
'nwa_letters_password' => NwaText(1126, 'Random letters'),
'nwa_lettersdigits_password' => NwaText(1119, 'Random letters and digits'),
'nwa_picture_password' => NwaText(6903, 'Format picture'),
'nwa_sequence' => NwaText(8001, 'Sequential numbering'),