If there were something going on internally which caused your usual sale notification email to not get sent, this would be happening to ALL of our merchants for ALL transactions, since ours is a shared, centrally-managed service where everyone uses the exact, same software and hardware at our end (rather then each seller getting "their own copy" of the software to run).
Since you are the only one reporting a missing Sale Notification email for just this one transaction, which otherwise appears to have been logged and processed normally by our system, we can only conclude the email did get sent and somehow got lost afterwards. Many ISPs have aggressive preliminary spam filtering that can block or discard some messages before they ever reach the recipient's account, so those wouldn't even show up in your spam folder, which is probably what happened in this case.