I’ve killed The Wandering Death about four times now, at least once on Normal, Expert, Torment I and Torment II. I still haven’t received its boss power.
Just to test if something was off, I went and killed Urivar and Herald of Hatred, since I was missing their powers too. I got both of those right away.
This leads me to believe that something is bugged specifically with Wandering Death. Would appreciate if this could be looked into.
Same here, it really sucks.
Figure it out — the main power must be on top, with all three below acting as modifiers.
+1 to the OP
I have every other boss power unlocked except Wandering Death.
I’ve killed it several times and the boss power still doesn’t unlock.
Just killed him again a few minutes ago and still nothing.
Hey, Seems like you may have read the post a bit differently than intended! 
To clarify:
We (Me and the others who posted) are unable to unlock the power to begin with, placing them out was never the issue, unlocking them at all is the problem/bug.
I saw someone in another post fixing it by activating the boss power on an alternative character, however, i cannot and have not unlocked it on any character.