If I understand correctly, Service NiFi works fine but from the Cloudera Manager things do not look promising this has to do with how CM monitors the NiFI service
I would start to at the logs directory under the NiFi processor directory on each host where niFi is running and look for file connectionMetricOutput.log
NNNNNNNN-nifi-NIFI_NODE/logs/connectionMetricOutput.log
On each Node where NiFi service is running, CM agent invokes nifi cli command get-nodes to get the health status of each node.
bin/cli.sh nifi get-nodes
If you find get-nodes command is not completing, you may want to further file a support case with us to look in detail.
I hope this helps.
Thank you