The world bosses seems to not drop any loot when he despawns. In the case that you are fighting him alone. It has happen to me twice now. It only happens when the boss goes into idle mode after being attacked. Like a few minutes ago when Kutum despawned he killed me and i ran back and saw him despawn and there was no loot. Three days or so ago I was at Nouver by myself and I hit him quite a few times and then just sat back and he went into idle mode. After he despawned he did not drop any loot. What I mean by idle mode is no one is attacking him and nouver he just moves back and forth and for kutum he stays in a frozen position.