I was running a monolith instance a few minutes ago and needed to destroy 2 Imperial Spires. Initially, there were no quest markers, and I never saw any artillery fire from them. After clearing the top half of the map and starting to clear the other side, about 3/4 of the way across the map, suddenly the quest markers appeared and the artillery fire started. Both Imperial Spires showed up on the other side of the map where I had initially cleared.
This is the first time I’ve seen it happen, so it may be too intermittent to track down. I attached the Player.log from just after I finished the quest if it helps any.
Player.log (2.5 MB)