I just noticed that myself yesterday for the First time. On official PVE server.

Anywhere up in that area. Including inside the Valcano, my follwers got stuck.
My horse actually ran away from me to the spot he got stuck in, like he was called to that spot.

Even after running accross the map dragging a Thrall they didnt teleport to me.
I had to use a Map room for them to rejoin me.

I will add, In this “Dead” zone of the map, NPCs in the area dont move either. They just stand there and allow you to attack them with out fighting back.