I am running a Volcano Orb Sorceror and just ran into the Mountain Aerie in my monoliths and noticed I was doing the animation, but no orb was spawning, just one single explosion.
This appears to maybe be either the final orb explosion, or the ground blast happening instead of an orb appearing, maybe? I went on to another map (Frozen Woodland) and it did not keep happening, but it did occur multiple times on just Mountain Aerie.
Attached is a screenshot I tried to grab, as I was unable to get a video, but you can still see the aftermath of the explosion and no orb being present.