I remember that when a customer subscribes paying via PayPal, then when you refund them via PayPal, Membership Pro recognize this action and automatically assign the subscription status from Active to Cancelled-Refunded.
Now this not happen. Is my memory wrong or there is some problem with this?
It should still work. Maybe for this subscription, PayPal could not send notification to Membership Pro for some reasons and it causes the error
Please monitor the next subscription (if happens) and in case it does not work, I will need to ask you to enable Log IPN Messages parameter in os_paypal payment plugin so that we have data for checking the issue further