I have a hobby business set up with PayPal Payments Standard. Each item button includes a confirmation/return to website https://www link. The confirmation page includes code to update my sales tracking database. I have found that sale notifications sent by PayPal with "Notification of payment received" as the subject line apparently are not returning customers to the confirmation page as my database is not updating. Sale notifications with "Payment received" as the subject line apparently are returning to my confirmation page as the database is updating as expected.
What is the difference between the notification types? Is one for PayPal account holders and the other for guest customers? Is there a PayPal setting I've missed that needs to be enabled/disabled for the customer to be consistently returned to my confirmation page?