Thank you @tony-josi-aws, this was indeed part of my issue. I implemented pxTM4C_FillInterfaceDescriptor as suggested. I left out EMACIndex for now as there’s only only one MAC. For others reference you also have to modify _process_received_packet() to fill in the endpoint. I cheated and used a global interface and passed that in like so:
/* Setup the event */
event.eEventType = eNetworkRxEvent;
event.pvData = cur_stack_descriptor;
cur_stack_descriptor->pxInterface = myPxInterface;
cur_stack_descriptor->pxEndPoint = FreeRTOS_MatchingEndpoint( myPxInterface, cur_stack_descriptor->pucEthernetBuffer );