If you run a multi-site DAG over subnets, it could happen, that you have anomalous database switchovers within your DAG. In the event log is triggered, that the FailoverCluster service has lost its heartbeat.
This happens because of the WAN connections latency. You could avoid this behaviour with the cluster.exe command:
cluster /prop SameSubnetDelay=2000:DWORD
cluster /prop CrossSubnetDelay=4000:DWORD
cluster /prop CrossSubnetThreshold=10:DWORD
cluster /prop SameSubnetThreshold=10:DWORD
Read this TechNet article for futher details.