I have never experienced issues with paypal either. However paypal does do some strange things some times with their API and IPN process so it is entirely possible. In my experience in the old days i have seen very unique issues with one customer and no issues with another depending on local environments. That does not say that im sure 100% that it is paypal, but it is certainly possible seeing what i have seen in the past.
I have also never had the license issue like others have had, every time i have ever purchased a plugin it has been in my account immediately.
The IPN which is the Instant Payment Notification method that paypal uses along with the website to process a sale (if they use that process) basically sends the sale record to the website after the sale is completed and verified. It happens only once, it can never be repeated, so the website gets one chance to get the information and that is all.. It is the same for all websites that use IPN, oxwall is no different. That is how IPN works.
The challenge is that only certain people have access to the code and it is up to them to find the issue and fix it regardless if it is a paypal based issue or other issue. I feel we have to focus on the things we have control over and not the things we dont.. I can tell you that requests have been made on this issue.