I made a thread about it but it's pending mod approval. Basically:
SM affects corruption, but not drain life, siphon life, curse of agony. Haven't looked at shadowbolt
Rank 4 drain life (level 38) drains 41 health at default. With imp drain life it's 45-46 (because 41*.1 = 4.1 which is 45.1) I have 3 points into SM so that should be an extra 2.46 damage. It should show 47-48 damage but it doesn't. It shows 45-46 on the tooltip.
All of you claiming it isn't broken: Have you tried it out yourself? I'm not going to recreate my thread in a single post, but I'll give you the same example with a screenshot:
http://i.imgur.com/bnL4gyh.jpgDefault Rank 3 drain life is 29 life drained. With improved drain its 31.9 but in this case the damage is rounded to 32. With 3 talents in SM it should be an extra 1.74 damage so 33.64. This is clearly not happening!
If this is somehow correct please explain, otherwise this is just a bug to me.