Assassinate doing little damage

Describe the problem. Be as specific as possible!:
I don't know if this is a known bug or not, but whenever I fully charge up assassinate, sometimes it does VERY little damage. The numbers are shown to be correct, but the monster's HP bar is still high up.

 Images of problem

It's done that since 4th job came out. It usually only happens if you change channels while still in Dark Sight (it might have done it while moving through maps as well, but I can't quite remember).