I guess the problem appeared since we added a new node and installed 6.3.x version of filebeat.
Yesterday I upgraded all the filebeats in the infrastructure, but the problem is still there.
I found a topic of Problem with transfer Filebeat 6.1.3 > Logstash 6.1.3 > Elasticsearch 6.1.3. There they recommend to mutate the fields in order to pass into the new index schema.