Yifirus seems to depend on server tick instead of his own lifetime

When you fight the boss, it seems like random, if his Breathtaking Water Pressure does damage or not.
Under "normal" circumstances, he's dead, before he can do damage to me.
But in some cases, he triggers this skill 2-3 times on me.

I tested it with always the same gear, weapons and buffs.
I don't see any other variable than the server tick, which you can somewhat track by watching the skill indicators on skills like Traces of the Cross Sword