Update request not updating documents in index

in Elastic 6.5.4 update request for a field does not update the field correctly.

The existing content is (partial content from doc)

"PrimaryEmail": {
"ContactPointId": 100000016065020,
"_secured": "Y",
"EmailAddress": "sendmail-test-discard@abc.com",
"OverallPrimary": true
}
},

Now there is a PUT request to update this content,

"PrimaryEmail": {
"ContactPointId": 100000016065020,
"EmailAddress": "sendmail-test-discard@oracle.com",
"OverallPrimary": true
}
},

So ideally after this request "_secured": "Y" should be removed. This is not happening.

Header of curl is

{
"update": {
"_index": "mm_d4e808fe-ede7-4e29-b5f5-f65ead14fa51_bb500161-b24d-4bde-afb5-f472cae272ae",
"_type": "documentType",
"_id": "300100179416462",
"_version": "5"
}"doc": {
...

We are doing a full update like referred here
https://www.elastic.co/guide/en/elasticsearch/guide/master/update-doc.html

What might be going wrong here due to which _secured is not removed. ?

Could you provide a full recreation script as described in About the Elasticsearch category. It will help to better understand what you are doing. Please, try to keep the example as simple as possible.

A full reproduction script will help readers to understand, reproduce and if needed fix your problem. It will also most likely help to get a faster answer.