I’ve noticed this only in Arathi Highlands, but that might only be because it’s one of the few places I still use my voidwalker.
Sometimes, he simply generates no threat, simple as that. Example today was Fozruk. I pulled him up and sent him in then started DPSing and Fozruk IMMEDIATELY shot up to 999% threat against me. I used Suffering to taunt him back to the voidwalker, popped one conflagrate and just stood there. Fozruk immediately focused me again at 300% threat and as the voidwalker attacked and I stood still, it never changed. Fozruk just sat there focused on me at 300%.
I’ve seen this elsewhere as well, particularly on Krix.