My Dark Iron Dwarf fury warrior just turned level 45. If I select either passive talents (inner rage or sudden death) no message shows up that I learned a new passive effect (like on other levels), and the new talent does not work. For example inner rage does not lower the cool down of Raging Blow, and Sudden Death still requires the target to go below 20). The active talent Furious Slash does appear to be working.
Does it show up in your Spellbook?
If it’s not working I would submit a ticket to Blizzard to have them investigate.
The Sudden Death talent is basically RNG, it has a chance to proc on abilities can be used on any target. Meaning you should probably setup a way to track this some how like with Weak Auras. I can see from your profile that the talent is selected and active.