Guild Wars 2

Get very close to the door to Cragstead and you will get the area.
Doing the storyline did not give it to me either.

I can confirm this happening quite often on my Guardian, on trident skills 3 and 4 are almost guaranteed to trigger Invulnerable (more than one time on the same mob). Tested this in Kessex with the Kraits around the witches lair. The spear skills do not seem to trigger it as often. I can provide video if you wish.
However, it very seldom happens on my ranger or ele.