In Hydra heads's description Head of Wrath has Vengeance meter tick on HIT. However Hp burn on other heads procs it, so does Ninja's Hailburn - even if you miss the Head of Wrath - it ticks for every burn activation. HP burn is one of the best ways to deal dmg to hydra, but with how fast it makes head of Wrath beef up - one can not rotate defensive buffs fast enough to protect from it's damage, especially if the burns are already up on other heads when Wrath is the new head to grow.

I believe it to be a bug as description talks about direct hits and getting killed when you mistakingly think you can control when Head of Wrath strikes back is just unfair.