snmptd probe is not sending hostname in hostname field of alarm.
The snmptd probe captures incoming traps and converts them into UIM alarms.
The probe has a feature of 'Name resolution' which however is dependent on IP to hostname resolution. 

UIM 8.x  or later

snmptd 3.22

In order to achieve this create entries for those hosts in etc/hosts.
This will help in achieving IP to Host Name resolution in environments where there is restriction for this lookup.
And post these alarms will start reflecting with Hostname in the respective field

snmptd (Simple Network Management Protocol Trap Daemon Monitoring)

Simple Network Management Protocol Trap Daemon (broadcom.com)