Trigger won't activated on specific scenario - BUG or a Feature?

[high-t]

Hi.

I have the following setup:

• Item looking for the work alert in a log file, but saves only what's written after the word alert:
  
  
  This means that this item's history only contains lines with alerts.
• A trigger set to fire when there is a difference between current and previous values:
  

Normally, this setup works as expected, but there is a specific scenario where the trigger wont fire, although the item does register the value in the history:
When a new server is launched, it auto register itself, and is then associated with the template containing the item and trigger.
At that specific scenario, when the very first alert appears in the log file, the item will register it, but the trigger won't fire. I suspect it this is because it is the first line in the item's history list and the

diff()

function is thus failing.
When the next line with the work "alert" appears in the log, the trigger will fire as expected. Furthermore: It I'm clearing this item's history and then push a new line with the word "alert" to the log file, everything works as expected.

Any outlook will be appreciated. Is this a bug? Is there a way of going around it?

Thank you in advance!
Amit.