Just to note, that screenshot is from the summer event in Ul'Dah, and i've been there.
The amount of active elements in that event tends to be gigantic, and by that I mean easily two or three times the max rendering capacity of the game (which is 40 elements). When I was doing it myself, in the final wave the other PCs were disappearing to give place to Bombards and I was easily hitting the cap on rendered elements.

Just stating that it might or not be you simply hitting the render limit, i'm unsure about the other cases but at least on the event case, that should be it.