We are using Hazlecast 3.4.1 post PRPC v7.1.8 and ideally this shouldn't be appearing. Are you able to see this consistently? Can you provide your topology with all the details about the nodes (like indexing, agents) whether they are on a same system so on.
I doesn't mean to check you any agent in specific, wanted to check the topology and if you use any specific node which is dedicated to run all the agents. If this is an intermittent issue, you will have to check internally if there were any network issues/firewall between the two nodes which is causing the disconnect between the 2 nodes.