Kibana visualizations & dashboards lost on upgrade from 6.6.1 to 7.0.1

I had deployed kibana-6.6.1-oss and created some visualizations, index patterns and dashboards in kibana.
My .kibana_1 was created as shown below:

I upgraded my ELK cluster to 7.0.1 (oss). All my data is retained but I see .kibana_1 index document count has decreased.

And on the UI i see all my visualizations & dashboards have been deleted.
Is the kibana index deleted on upgrade?

In kibana logs after upgrade, i see this

{"type":"log","@timestamp":"2019-07-04T10:10:25Z","tags":["info","migrations"],"pid":1,"message":"Creating index .kibana_1."}
{"type":"log","@timestamp":"2019-07-04T10:10:26Z","tags":["info","migrations"],"pid":1,"message":"Pointing alias .kibana to .kibana_1."}
{"type":"log","@timestamp":"2019-07-04T10:10:26Z","tags":["info","migrations"],"pid":1,"message":"Finished in 1295ms."}

How can i retain the data stored in .kibana_1 index from previous cluster?

Did you run the Kibana upgrade assistant before upgrading? It should point you through all the steps required to keep all your documents.