PayPal IPNs not processed if "receiver_email" != "business"

By default, PayPal IPNs (Instant Payment Notifications) are posted to CiviCRM with the "receiver_email" parameter equal to the "business" parameter: both are the receiving PayPal account's primary email address, which should be equal to the account email set up in the CiviContribute payment processor configuration.

But if you log in to PayPal and update the email address that should receive payment notifications, that address becomes the value of "receiver_email" in the IPN sent to CiviCRM, while the "business" parameter continues to match the account email set up in CiviContribute.

The result is, CiviCRM appears to correctly process the IPN (it responds to PayPal with an HTTP 200 and logs no error), but it seems to record no data at all. If someone has paid as part of an online event registration with payment, for example, their registration remains "Pending from Incomplete Payment" indefinitely, although PayPal has processed their payment, sent an IPN, and received an "HTTP 200 OK" from CiviCRM.