Upgrade to 3.5.8 which contains the fix for the above reported bug.
We are using ZK 3.5.7 which contains the following major bug in ZK. It was fixed in version 3.5.8
" In a cluster with three nodes, node3 is the leader and the other nodes are followers. If I stop node3, the other two nodes do not finish the leader election. This is happening with ZK 3.5.7 "