Elastic-Search Index not deleted but received ack. true

Delete index API of elastic has responded with acknowledgment = true but the actual index is not deleted.
We are using ES 5.3 Java TCP Client. We have 4 data nodes and 3 master nodes. In the elastic-search logs, we could not get an exception.
Overall we have approx.. 150 indexes with 2 primary shards and 1 replica. Each data node has 16 GB.

@elasticsearch -- any update on this?

What does your node configuration look like? Do you have any non-default settings? Does the request return a successful response?