This is actually working as intended, but the wording is a little confusing.
What it says > [conditional use/after rank up] Do [damage: 50% of target's MAX HP] to all enemy units. Decrease your Gowen LV by 1.
What it means > [conditional use/after rank up] Do [damage: 50% of each individual enemy units MAX HP] to each individual enemy unit. Decrease your Gowen LV by 1.
Example > The enemy has 2 units on the field unit A has max HP of 40 unit B has max HP of 60. Unit A takes 20 Dmg while unit B takes 30 Dmg.
I am taking talking to myself I know, but one day you may all jump on the burning bandwagon.