Scratch that. It actually makes sense.
One of the advantages of HISMs is per instance culling. As soon as they leave frustum, they’re culled - you’re experiencing the same as my almost working blue sphere animation.
Not sure whether this is a bug or HISM limitation 
It seems to affected by the way the instances are originally ordered:
As can be seen above - one of the sides is barely affected as those indexes are neatly ordered ascending.