Random completed PayPal transactions are not being updated in CiviCRM and the contribution records are incorrectly left as "Pending." I have worked with PayPal to confirm that all IPN notifications are being successfully sent from their end. CiviCRM's logs do not show any errors; they simply do not show that they received any notification from PayPal.
To make matters more confusing, when I resend IPN notifications through PayPal, they are then correctly received by CiviCRM the second time around and the contribution record is updated.
Any ideas on how to rectify this situation? I cannot find any pattern as to which notifications go unreceived and which contributions are successfully updated. I am on WordPress 5.5.3 and CiviCRM 5.30.1.