Hi @Billias,
In Metricbeat 6.3.0 the host field is defined as an object, this can break other uses of this field, as when using beats with logstash, as in this topic: Logstash errors after upgrading to filebeat-6.3.0. Take a look to the previous topic to see if any of the commented workarounds works for you.
We are currently working on improving the docs to give more visibility to this breaking change.