As the title says.

I get the 1% HP per stack against trash mobs as advertised, but I'm not getting the 1d6 per stack bane damage against bosses which is also advertised. I've even tried just standing and blocking, letting the aura of purification do its work of putting stacks on the enemy, but I'm not getting the damage against red-named (haven't really had the opportunity to pay attention to purples).

Is it just bugged or are there some restrictions in place? Like only against undead and/or evil outsiders or something?