Steps to reproduce:

1. Create host with IP as hostname and DNS name as visible name
2. Run the following api query:

   {
   "jsonrpc":"2.0",
   "method":"trigger.get",
   "params":{
   "output":[
   "triggerid",
   "description",
   "priority",
   "hostname"
   ],
   "filter":{
   "priority":[
   "3",
   "4",
   "5"
   ],
   "value":"1"
   },
   "active":"true",
   "sortfield": "hostname"
   },
   "id":1,
   "auth":"{{zabbix_token}}"
   }

Result:
You will see the visible name and not the hostname for all triggers and their hosts. 
Expected:
One would hope that you would see the hostname which in this case is the IP address.

 

I realize that the trigger.get query does not have the a hostname object associated directly with it however the query runs fine so either it should not be allowed or should actually return the correct value imo.