I have managed to get 4 total. 3 are from world bosses and 1 today from an emissary cache. 1 each on 4 toons and nothing on the last.

I don't think there is bad luck protection involved. I play a lot, lots and lots, and its just the more you play the better your chances. Plus maybe a bug that effects some people and they get lots of them. Blizzard says the bug is fixed, but they suck at math so probably still have a decimal point in the wrong place somewhere.