Here's a way to bring it back. If you spec in to

Here's some links to proof.
Mutilate with Dirty Deeds without Poisons:

Mutilate with Dirty Deeds with Poisons:

Mutilate Hits with Dirty Deeds increase: 483 (No Poison) ---> 1147 (Poison) = ~50% Increase
Mutilate Crits with Dirty Deeds increase: 726 (No Poison) ---> 1694 (Poison) = ~47% Increase
If the sample sizes were larger it will balance out to the 50% increase, but it definitely proves that its doing more than the +20% that its supposed to do.
So Rogues take your daggers back out and wipe away your tears. Its time to spec into Mutilate again, just to remember to spec Dirty Deeds along with it. It might take a couple of re-specs to make it bug, so try re-specing a few times if it doesnt work.