Died from fire of Iscariot in HE5 with alacrity

Hello. I died 3 times while being on fire from Iscariot firebeam attack in Hell Eternal 5th boss fight. I was properly running every time. Due to random target of attack I was able to notice only this:

  1. if I get firebuff and I don’t use any alacrity weapon then everything is fine.
  2. if I get firebuff and I have hammer with alacrity but I don’t switch weapons while running then everything is fine. Doesn’t matter if my current weapon have alacrity or not.
  3. If I get firebuff and I switch weapons while running I die. For example I was attacking with shotgun, got fire, started to run and switched to hammer to run faster - I die from DoT. It did not help to switch back to SG.

Thank you.