I have Debuff spells cast after attack (Pestilence and Poison Gas). When I attacked and killed the enemy, they both proc’d one after another. All extra debuffs applied as normal, but the damage from the perk never showed, even in the battle history; but it did show that my attacker, who cast the spells, has healed too many times this turn (It has leeching from trait, as well was full hp). I have Final Excretion that proc’d after the debuff spells were cast due to my attacker killing the enemy, who had debuffs, outright.
I have noticed it shows the damage when a buff is applied once when my monsters attack and apply a single debuff from their artifact.
Did a test with the same set up and spells but manually cast them, and the unholy night perk procs damage. Also shows correctly in battle history