I have noticed this and done a bug report in game on it. This seems to be caused, from what I can tell, by combat starting prior to a weapon actually making contact. For instance with a ranged weapon I have not seen opening strike go off a single time. I fire, combat begins, buff falls, damage is dealt and no vulnerability is applied. This was working fine as of bwe 2 (last time I played ranger) so it should be perfectly able to be reverted.