At the moment, if a mob isn't aggroed, you can't assassinate them because your dmg applies before the Assassinate attempt, aggroing the mob thus not letting you kill it. If you want to solo something, you just can't use it.
The main feature of the assassins isn't working properly at the moment. Pretty a huge bug I'd say.
Fix this ASAP.
Re-think about this patch, delay it, do something but please fix another 5-10 things (specially Epic destinies bugs) before putting this to live. I've never been so unhappy with a Patch, really but this is outrageous (is this the right term? Pardon my english ).
I can't check in game at the moment but I guess if it's not in the notes, it's not fixed.