Did you check weather the username is different from the email? In Sylius the user gets the same username as his email address as you can see in
Sylius\Bundle\CoreBundle\Model\User
class User{
...
public function setEmail($email)
{
parent::setEmail($email);
$this->setUsername($email);
return $this;
}
public function setEmailCanonical($emailCanonical)
{
parent::setEmailCanonical($emailCanonical);
$this->setUsernameCanonical($emailCanonical);
return $this;
}
}