Many of the Infestation style POIs have the bound spawn close to other mobs in the game. Quite often the bound will immediately agro onto the player once you begin to attack them. However, if you walk close to a POI and then get involved with say a Fabled Creature, then before you can attack the bound they have already begun to wander away, or are now chasing other mobs making it difficult to impossible to complete the Infestation POI.
My suggestion would be to have the absent bound respawn, or return to the POI from which they spawned if they move more than a given distance from the POI (maybe 50 meters).
On reloading into the game the POI's progress seems to have reset and I am now able to kill the bound.