Caster DPS Malevolent 3 set bonus tooltip claims +50 damage to subsequent spells. However, in its current state, after proc it is consumed by the next spell that hits the targe. So firstly, it is only benefiting one spell, which, seems to be a bug, considering it states “… cause your target to take up to 50 increased damage from subsequent spells”. Secondly, it is being consumed by other players. Finally, it is currently so underuned, players are using gnomer tier sets, or 2/3 set, as they perform better. I do not think this is intended.
As for a fix, the 3 set should work as the tooltip states, after proc, your target takes +50 damage from you only, until it is dead. If this is causing issues, a temporary or permanent spell damage bonus would be a safe secondary fix.
This would make players want to use the new tier set and make the achievement of getting it feel better.