Mapping fields cannot be replaced during template composition

Hi, I'm trying to setup a template index composed of some component templates. One of the component templates creates new fields mapping on an existing object, and this raises the following error "mapping fields [tcp] cannot be replaced during template composition"

Here are the steps to reproduce:

    PUT _component_template/tcp_service
    {"template": {
                "mappings": {
                    "properties": {
                        "tcp": {
                            "properties": {
                                "tcpService": {
                                    "type": "keyword"
                                },
                                "udpService": {
                                    "type": "keyword"
                                }
                            }
                        }
                    }
                }
            }
        }

Which responds

    {
    "acknowledged" : true
    }

And then create the index_template

    PUT _index_template/data
    {
       "index_patterns": ["data_*"],
       "composed_of": ["tcp_service"],
       "template": {
              "aliases": {
                     "data": {}
              },
       "settings": {
           "index": {
               "number_of_shards": "2",
               "number_of_replicas": "1"
           }
       },
       "mappings": {
           "properties": {
              "tcp": {
                   "properties": {
                       "dstPort": {
                           "type": "keyword"
                       }
                   }
                }
             }
          }
       }
    }

And the answer

        {
      "error" : {
        "root_cause" : [
          {
            "type" : "illegal_argument_exception",
            "reason" : "composable template [data] template after composition with component templates [tcp_service] is invalid"
          }
        ],
        "type" : "illegal_argument_exception",
        "reason" : "composable template [data] template after composition with component templates [tcp_service] is invalid",
        "caused_by" : {
          "type" : "illegal_argument_exception",
          "reason" : "invalid composite mappings for [data]",
          "caused_by" : {
            "type" : "illegal_argument_exception",
            "reason" : "mapping fields [tcp] cannot be replaced during template composition"
          }
        }
      },
      "status" : 400
    }

Am I trying to do anything impossible?
Is the only way to accomplish this to include component_template fields in index_template?
Can't Objects be updated with component_templates?

Kind regards, Manuel