PayPal IPN Notification Inconsistently Received
As an update to this thread, the issue was identified as a bug in the PayPal integration (https://lab.civicrm.org/dev/core/-/issues/1931) and was resolved in an update pushed by CiviCRM Admin Utilities 0.8 released on 20 November 2020. Payments have been closing perfectly for us since applying the patch.