I don't disagree, but in this case I was just sitting in wherever-that-was working through the quest mobs. And when a chain ended I was like... there's something missing from this picture.....
Finished the quest with the pet offscreen, zoned, pet still offscreen, pulled a mob, no difference, resummoned and new pet appeared in the correct place.
Thinking about it though it's possible that my pet attacked an enemy before the enemy had popped fully? (Potentially something that got the edge of an AE as it spawned?) And then got stuck? That's a bit enthusiastic of my pet but..... plausible!