Currently, when using the quester, the "target" object (either NPC or gameobject) is being blacklisted, if there are more than the maximum number of units (in your settings) near it.
This is a problem, because it completely breaks these quests. It's worse for game objects, because most of the time they are being blacklisted when no unit is around at all.
But it's also very annoying for "boss" type NPCs. It makes questing very bugged and annoying.