You would only receive a copy of the thank-you email if you enabled "Receive a copy of email sent to buyer" in the product's settings. The fact you received your copy confirms our system did send a thank-you email to the buyer, as the "Receive a copy..." setting merely adds you to the Bcc: line on that same message. Ask your test buyer(s) to look in their spam/junk/bulk mail folder to see if their thank-you message wound up in there, and be sure to mark the message as Not Spam to help train their email provider's spam filters.