>>536152958
Ksenia applies a burn buff to the first 1-2 doll(s) that deal targeted damage this turn. Which in turn triggers this fixed key, removing the debuff.
If Nikketa's first turn is an AoE attack, she doesn't trigger Ksenia's buff then. So on her second turn in a round she ults, dealing targeted damage and getting the buff and cleansing the debuff then, refreshing the buff from the boss mechanic.