AWS Elasticsearch frequently getting into yellow state

I have AWS Elasticsearch running with 3 Master nodes(C4.large) and 10 data nodes(C5.large). Recently I am experiencing frequently that my domain gets into yellow state for some time around 30 mins and then i do nothing it changes to green.
when i used the query GET /_cluster/allocation/explain?pretty this is what i see

{
  "index" : "lgst-",
  "shard" : 4,
  "primary" : false,
  "current_state" : "unassigned",
  "unassigned_info" : {
    "reason" : "NODE_LEFT",
    "at" : "2021-01-06T13:15:38.721Z",
    "details" : "node_left [**************]",
    "last_allocation_status" : "no_attempt"
  },
  "can_allocate" : "yes",
  "allocate_explanation" : "can allocate the shard",
  "target_node" : {
    "id" : "****************",
    "name" : "********************"
  },

I couldn't understand what does it mean and how do i over come it. Any help please would be appreciated.

Solution

Looks like you are using spot instances in your cluster and cause for this is that nodes in your AWS is not stable as shown clearly in the unassigned_info

"unassigned_info" : {
    "reason" : "NODE_LEFT",
    "at" : "2021-01-06T13:15:38.721Z",
    "details" : "node_left [**************]",
    "last_allocation_status" : "no_attempt"
  },

I would suggest chaning the instance types if you are using ec2 spot instances and check why nodes are getting disconnected in your cluster with AWS support.