Upgraded to Elasticsearch 7.8.0 and now date fields are in fields key instead of _source

Hello,
I've upgraded my cluster from 6.8.0 to 7.8.0. This is my template:

{
    "index_patterns": [
        "example-*"
    ],
    "settings": {
        "number_of_shards": 1,
        "number_of_replicas": 0,
        "refresh_interval": "20s",
     }
    "mappings": {
        "dynamic": false,
        "properties": {
            "@timestamp": {
                "type": "date"
            }
       }
   }
}

When I save my document this is what I get:

"_index": "example-000001",
  "_type": "_doc",
  "_id": "gV2ZAHMBqN3Ho-B9PIfj",
  "_version": 1,
  "_score": null,
  "_source": {
    "@timestamp": "2020-06-29T15:01:49.000Z"
  },
  "fields": {
    "@timestamp": [
      "2020-06-29T15:01:49.000Z"
    ]
  },
  "sort": [
    1593442909000
  ]
}

What is this new fields key I see on my documents? I hadn't it on my old elasticsearch 6.8.0.

I did some testing and I've found that every field I define in my template using date ends up inside fields key on the document

Does anyone have an hint on how to resolve this issue?