It depends. Is the attack they're getting a vengeance strike from Repel Boarders? I think first strike goes against a targeted attack, not one of the freebies, but I could very easily be wrong. I often am.
I noticed this behavior with Ratbeard. He has Hold the Line, First Strike 2, and Repel Boarders.
An enemy would approach, Hold the Line would trigger, Repel Boarders would trigger, but First Strike would NOT trigger before the enemy got an attack.
Bug?
Just based on my review of the talents, it sounds like a bug.
There are some attacks that (by design) can defeat than First Strike-- either because they are too fast (Riposte) or too sneaky (Flanking) for the First Striker to respond.