I looked over the code stock code for 6.2 b2b 13.2 that handles this and did not see anything wrong at first glance, except for the code to send an email to the store owner. If the customer is getting the welcome email then they should be getting the code with it, if enable and setup. I did not do a test to double check to see if it is actually working.
if you created the voucher, then entered the code into conguration > my store > Welcome Discount Coupon Code. Then it should be picking this code up. This does use the tep_mail_function, and the link for the coupon code is a html link, so configuration > email > Use MIME HTML When Sending Emails needs to be set to true so the link is not removed by the tep_mail function.
There may also be other reason for it not to work, Like some add on contribution for create accounts that is interfering with this code.
Tom