Unable to apply new index template - elasticsearch

Question

I am currently trying to update an index template on Elastic Search 6.7/6.8.
Templates are stored in the code and are applied each time my API starts.
There are no errors, the request returns 200.
For example, here is a template i am currently using:
{
"index_patterns": [ "*-ec2-reports" ],
"version": 11,
"mappings": {
"ec2-report": {
"properties": {
"account": {
"type": "keyword"
},
"reportDate": {
"type": "date"
},
"reportType": {
"type": "keyword"
},
"instance": {
"properties": {
"id": {
"type": "keyword"
},
"region": {
"type": "keyword"
},
"state": {
"type": "keyword"
},
"purchasing": {
"type": "keyword"
},
"keyPair": {
"type": "keyword"
},
"type": {
"type": "keyword"
},
"platform": {
"type": "keyword"
},
"tags": {
"type": "nested",
"properties": {
"key": {
"type": "keyword"
},
"value": {
"type": "keyword"
}
}
},
"costs": {
"type": "object"
},
"stats": {
"type": "object",
"properties": {
"cpu": {
"type": "object",
"properties": {
"average": {
"type": "double"
},
"peak": {
"type": "double"
}
}
},
"network": {
"type": "object",
"properties": {
"in": {
"type": "double"
},
"out": {
"type": "double"
}
}
},
"volumes": {
"type": "nested",
"properties": {
"id": {
"type": "keyword"
},
"read": {
"type": "double"
},
"write": {
"type": "double"
}
}
}
}
},
"recommendation": {
"type": "object",
"properties": {
"instancetype": {
"type": "keyword"
},
"reason": {
"type": "keyword"
},
"newgeneration": {
"type": "keyword"
}
}
}
}
}
},
"_all": {
"enabled": false
},
"numeric_detection": false,
"date_detection": false
}
}
}
I'd like to add a new keyword field under the properties object like this :
"exampleField": {
"type": "keyword"
}
but it seems the template is not applied to existing indexes.
When data is inserted into a specific index which use the template, it is stored like this:
"exampleField": {
"type": "text",
"fields": {
"keyword": {
"type": "keyword",
"ignore_above": 256
}
}
}
because the template has not been updated beforehand.
I would expect it to be like:
"exampleField": {
"type": "keyword"
}
in the index and in the template.
Does someone have any idea on how to have this result?
Thank you, Alexandre.