If it passed paypal then it should just be truncated if it was a data length issue with your database. I doubt that was the problem. That theory would make sense if it failed paypal checkout as well, but that doesn't seem to be the case.
Maybe your best option is to start logging the information returned from paypal on the pp_confirm.php page. We could store the returned information in .txt files associated with the order timestamp so that if one ever failed we could look up the result. You could manually clear out the folder monthly when there are no issues, but have a log if you ever have an issue. That would give us something concrete to go on.
Do you know if they got a success or failure page on your site?