The Most Horrific Vision of Orgrimmar

Confirmed as remaining BUGGED - 11/06/26.

I'm writing from the perspective of the 5 mask trial and how to avoid the bug

During the Valley of Spirits with Zekhan. The NPC will run to the end of the tunnel and stop just prior to the first totem you need to disable.
He'll stand there and won't move.

This means after you disable all totems, activate the sub-boss and kill it, Zekhan will not be in LOS and the quest will fail to update destroying the successful completion of the achievement.

Workaround: what I did...

Clear all the mobs between you and the sub-boss.
Deactivate all the totems to activate the sub-boss.
Taunt (or body pull) him back to Zekhan standing in the mouth of the tunnel.
When you are confident the sub-boss is within a two/three meters of Zekhan destroy the sub-boss.

Notes: Took me 4 attempts to...
Identify it was even a problem(!)
Troubleshoot it
Then complete a run where the workaround succeeded as described above.