I've configured a host with 2 SNMP interfaces and selected the second one to be the main interface. In the database this is set correctly.
According to https://www.zabbix.com/documentation/3.0/manual/api/reference/hostinterface/object the main interface should be used by default. But when I create a SNMP item the first interface is selected by default. In my understanding the second interface should be selected by default because it is defined as main interface.