Receiving certain tasks (as shown below) the effects of slayer helmet are not working on the bosses, but yet we receive the slayer xp for the kill. So for example if i got a greater demon task of 100 and decided to go K'ril Tsutsaroth, the effect of slayer helmet (damage bonus) isn't working (if it's in multi-zone area and there's more people, i've got a slight disadvantage, yet i've got a task that corresponds to the monster). What i would like to suggest is to fix the slayer helm affect, so the damage boost would work on both monsters if they are related to each other as shown below.
Black demons = Demonic gorillas
Greater demons = K'ril Tsutsaroth
Black dragon = King black dragon
Hellhounds = Cerberus
Abyssal demon = Abyssal sire
Leave an opinion if you can, i tried to explain it well, hope it makes sense, thanks !