2 Years ago this issue was reported and i am experiencing it now.
Hey Sikay,
If this does happen again for you, please do report it as a bug in-game so it can be looked into further and retry the scenario. Do note that he does not spawn until the door to the Siegebreaker itself is opened.
Have a few seconds to spare? Let me know how I’m doing!
Alrighty boss. Thank you. Also the workaround is leaving the quest scenario, resetting the instances and then make sure that your Siegebreaker spawns in the pit, if he doesn’t, repeat till he does.