I am almost certain that the witchcraft power Aura of Siphoning is currently healing enemies. I am playing on my poison Spiritborn and stacking as much poison DoT as possible along with other modifiers. Currently in a T3 witchtide and for some reason the mobs were just not dying in a reasonable timeframe, despite my gear and damage output improving: so I began to investigate. Checking my spell tooltips revealed that my Touch of Death, along with other damage over time spells, is “hitting” enemies for -2 billion damage over 72 million seconds. I also noticed that enemy life bars were not getting the DoT visual effect when stacking. As soon as I removed the aura, boom, all the mobs begin to die and take DoT including the correct life bar visuals. I have not seen this reported anywhere else but it seems pretty game altering and should be addressed asap, akin to the Hex of Whispers fix. Until then fair warning about using that witch power. Please chime in with any other testing/findings.
P.S. The tooltips only update in combat and change to normal working numbers as soon as you unequip the aura. The bug is most likely doing something with the % heal over time that the power is supposed to be giving to the player.
UPDATE
So with even more extensive testing, I found that the above bug occurs when equipping the Vulture Occult Gem that makes your DoTs roll 50% faster. For some reason as the DoTs ramp this will cause the underflow and end up healing mobs. On bosses it will stack so hard the boss will take just about no damage and then instantly be killed. Not sure how efficient that is but overall this makes killing some enemies an absolute pain. Hoping this gets addressed soon. For now I have only tested this interaction on the Spiritborn.