Lagon Cannot Be Targeted or Damaged by Drain Life

This is in the campaign of online Multiplayer, Lagon does not get damaged or receive skill side effects (like damned stacks) from Drain Life. I have tried using the skill basically everywhere in the arena, and all across the boss himself and his various limbs; alas, Drain Life is useless in this fight. However, Lagon is damageable by other sources (Rip Blood and Spirit Plague and the latter’s debuffs/plague in my case).

I am not sure how to relay my current state in the game, so I made a build with my skills/passives:

Many thanks for your support!

This happens in the Ending the Storm monolith as well. Drain Life does absolutely nothing against Lagon which is a bummer as that would be the main damage spell in my current build.