I have a trigger like proc.num[process.exe].sum(*720)*}=0 that seems to trigger unexpectedly. 

The item/trigger associated with this process.exe will be found via a discovery rule.

The count function is attached to the proc.num so the 720 should be from the time the process is discovered but in my Zabbix it will alert within the first 2 minutes of the process being discovered based on the 60 second poll interval. 

Is this a bug ?

The documentation says "evaluation period in seconds or in latest collected values (preceded by a hash mark)". So 720 seconds is the evaluated period from the time the process item was added and then from that point... if 720 seconds of evaluated data the sum() = 0 ? This is how i interpreted the function. 

But it seems to just be 720 seconds regardless of the items actual life.