I'm a corsair and was doing HP quest at Eastern China Shaolin Temple Floor 7. I was trying to use Ice Splitter to freeze Small Gold Monks and Giant Gold Monks. Some of the mobs would freeze and some would not.
Usually this get fixed by running the game in an older windows compatibility mode. Currently this is not recommended due to it causing crashing issues though.
I am also seeing this in Rose Garden with Frost Prey/Blizzard (Marksman skill). For me it's consistently happening with all mobs, not just some. I also have Windows 7 compatibility mode back on now (I switched to the recommended fix of disabling the server clock display instead).