@kkau 

Sorry to hear that the embedded documentation was not accurate for the port property.  
I filed an apache jira to address that issue:
https://issues.apache.org/jira/browse/NIFI-8536

Best path forward is following NIFi recommended best practice where each NiFi node in your cluster is on a unique host.  This would allow each node to bind this reporting task to the same port.

Having multiple hosts on same server will present issue/challenges with any component (processor, controller service, reporting tasks, etc) that create a listener port.  While some may you may be able to work around with Variable Registry suggestion I shared, others like this use case will not.

Thanks,
Matt