Stormheim’s introduction questline is unable to be completed currently. The issue has been reported for similar matters pertaining to other vehicles/cannons, but there seems to be an issue with firing and aiming the cannons once you reach stage 2 of Greymane’s Gambit.
The cannons will constantly do what seems like backfiring (the shots explode at the actual gun) unless you’re aiming at a specific area in the bottom right corner of the backdrop wall. Unfortunately, this allows you only to kill 12 to 14 Forsaken Dreadwings of the required 30.
At first, I thought it was a UI issue. Still, upon disabling all addons, deleting the cache folder (I don’t think this would do anything), dropping the quest/scenario and restarting it (multiple times, including waiting 45 mins before continuing), the issue still exists. I also adjusted the vehicle aiming both up/down and increment/decrement, and nothing worked to fix the problem.