I have the same problem, however I have a custom registration process which generated the email, etcs and still the default email is being sent.

Below is the code I am using

//Set required information to the UserInfo object
UserInfo objUser = new UserInfo();

objUser.PortalID = this.PortalId;
objUser.IsSuperUser = false;
objUser.FirstName = txtFirstName.Text;
objUser.LastName = txtLastName.Text;
objUser.DisplayName = txtFirstName.Text + " " + txtLastName.Text;
objUser.Email = txtEmail.Text;
objUser.Username = txtEmail.Text;

UserMembership objMembership = new UserMembership();
objMembership.Approved = true;
objMembership.CreatedDate = DateTime.Now;
objMembership.Email = txtEmail.Text;
objMembership.Username = txtEmail.Text;
objMembership.Password = random_string;

//Assign membership object to the UserInfo Object
objUser.Membership = objMembership;

//Register User
UserCreateStatus result = UserController.CreateUser(ref objUser);

//Check status
if (result == UserCreateStatus.Success)
{
// Save details to database and send necessary emails
}

This has happens in the latest version (7.3.4) because we have used this registration process on several sites over the past 2 years, on older version without having any additional emails.

Please help