I think it works as it suppose to be. here the source
http://wow.gamepedia.com/Shadowburn
It says "Now cost Burning Embers instead of mana. Deals Shadow instead of Shadowflame damage. Regains mana after 5 seconds, and gains Burning Ember on kill instead of Soul Shards"
And then blizzard buff it in 5.2 ""Shadowburn now properly generates 2 Burning Embers after killing a target that yields experience or honor."
So, it cost an ember, if it kill the target, you get 2 embers back
Nothing change since 5.2
this is a fixed for warlock positive bug. I hope that help