Weakness Trigger on normal (EP) mob in Dynamis?

Was fighting an orc in Dynamis, and I cast Dia II on it. A blue stagger animation appeared above his head and the message "Your attack staggers the fiend!" came up in the log.

Platform:
Xbox 360

ISP:
Comcast

Type of Internet Connection:
Cable? >.>

Internet Connection Speed:
Not sure.

Date & Time:
Friday, May 13th at 7:20ish AM PST.

Frequency:
First experience with this.

Character Name:
Zatias

Race:
ElvaanF

World:
Pandemonium - Asura

Main Job:
Red Mage

Support Job:
Scholar

Area and Coordinates:
Dynamis - Beaucedine at the former "NM lake" near the Mega Boss.

Party or Solo:
A party of 2; me and a Ninja.

NPC Name:
None.

Monster Name:
Vanguard Bugler

Steps:
1. The Ninja pulled the monster with provoke.
2. We began fighting it.
3. I casted Dia II on it. A blue !! appeared above its head and my log displayed "Your attack staggers the fiend!" with no other message (no "unable to ___" message).
4. The ninja and I began spewing random profanities in surprise.

Would like to know if it's a bug or intentional. If it even was intentional, the !! was not yellow, and this happened on Vanadiel WINDSDAY. I be confused D: By the way, the mob dropped nothing when we killed it.

Hey there Zatias!

Thanks for taking the time to fill out all the details we require. We appreciate you letting us know about that odd occurrence. We will gladly forward the concern to be investigated. Any updates will be posted within the forum as they become available.