This is not my find, Sebleb from WoW Secret Finding stumbled upon it by accident.
But I figured out how this bug is triggered

First of all: The quest you need for this is https://www.wowhead.com/quest=47229/bulwark-of-torcali#
This quest, is the Blood Gate storyline, which you can then accept when you have completed the storyline in Zuldazar and Nazmir. (This is a Horde quest)

This wooden post must stand between you and the vehicle, as I am standing there (a little too far to the right or left could no longer work) if you click on the vehicle, you will not mount it, you will be in the phase with the Risen Gargantuan and can then tame them

The WQ Version
The World Quest that is required is called: Old Rotana
Basically it's simple, activate your N'lyeth, Sliver of N'Zoth and fear Old Rotana, until it's on the wall of the house like on my screenshot, maybe one of the other objects / buildings in this World Quest area will also work.
I recommend using the Freezing Trap to hold Rotana where you want it, as well as Binding Shot.
If she walks somewhere she shouldn't, shoot at her, but be careful, when she is no longer under the effects of Scare Beast or Freezing Trap she will run back to her place.
