Glad you could reproduce, and that you saw the same behaviors that I did.
For more testing - perhaps it is very much tied to the map terrain.
I have noticed that the map has a couple of different layers. There is a graphical rendering layer (what we see), a player collision layer (what our character can walk over, climb on, fall, etc.), and an NPC collision layer (the layer where NPC pathing happens). The presence of these layers (and their occasional lack of congruity) becomes obvious in a number of different trouble spots. Sometimes when climbing, your character’s toon will go into the wall (render layer larger, or overlapping the collision layer). This can happen in the reverse, also, where you look to be climbing on air about 6-12 inches off of the cliff face (collision layer larger, or overlapping the render layer).
I would wager than when you “leash up,” the game is doing a calculation of the distance and the terrain between you and the thrall. If there is a rough hewn area, or if for whatever reason the NPC collision layer is jacked up, the binding will need to find a “smooth,” walkable path between you and the thrall.
What we are witnessing when the binding goes wonky is a longer pathing or route to the thrall. It would play to reason that if the path is too far for a walkable NPC surface, the binding leash-up encounters an exception, and fails. Player and game client say bound, server and NPC don’t bind because the path is too long, or does not work, and thus fails. The exit of that failure removes the KO’d thrall from the map.
I could be entirely off base. But what I have described is pretty consistent with my experiences. As you said, Barnes - hooking up the leash on a flat surface with little to no relief seems to reduce the hook up risk.