[Mivereous]

You've been poisoned and not experienced this?

Also,

I guess the reason this has never been noticed before is because the Kensei are generally reasonably fast. That or the wrong direction was being faced. But I played with the Desperado the other day and they were well aware of the facing issue, so I highly doubt that's the problem.

Seeing as how they would have had to Basic Attack or use a Kensei skill to get the damage from Sheath Sword (which can be faced towards the enemy), they would have already been facing the enemy. I've played with Desperado, also, and we both know about the facing issue.