Why can't Dgraph work well when I kill one node in the cluster

thx

And I have a little confuse in the concepts between Replication and Group.
1 Is data only replicate between nodes which belong to the same group?or it can replicate across groups?
2 How should I config the nodes for cluster to keep it working when partial nodes fail?
nodes are

  1. partial of one group
  2. all of one group

I have seen a passage on the official websit that

Replication and Server Failure
Each group should typically be served by atleast 3 servers, if available. In the case of a machine failure, other servers serving the same group can still handle the load in that case.