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I am trying to create a CSV report based on a saved search that contains fields like @timestamp, A, B, C.D, C.E, F, etc. But the generated file only contains @timestamp, A, B, F - in the report all complex fields are missing.
In fact - the fields that mapped as object in my index are missed.
Why did this happen and how can I fix it?

Kibana v 7.10.0 from docker-image "amazon/opendistro-for-elasticsearch-kibana:1.12.0"

For example:

   {
        "_index" : "my_index",
        "_type" : "_doc",
        "_id" : "789be064-0758-4245-8ca1-6570df3bb5a7",
        "_score" : 1.0,
        "_source" : {
          "status" : "alert",     -   simple field = string
          "issue_uid" : 123,   -   simple field = number
          "created_at" : "2020-09-21T05:02:48.000+03:00",    -   simple field = date
          "alerts_count" : 2,   -   simple field = number
          "labels" : {       -   complex field = object 
            "kubernetes" : "cluster.k8s",    -  part of complex field  -  will be missed from CSV-report
            "statefulset" : "sts1"  -  part of complex field  -  will be missed from CSV-report
          }
        }
    }

This issue is submitted by @uzhinskiy
opendistro-for-elasticsearch/kibana-reports#363