See for example https://civicrm.wikimedia.org/damaged/26130
The refund message is tagged with gateway paypal_ec, so the refund consumer is looking for something with trxn_id=PAYPAL_EC <parent_id>. That doesn't exist, but RECURRING PAYPAL <parent_id> does.
This recent refund should have been marked paypal_ec:
https://civicrm.wikimedia.org/damaged/26867