Issue - Candlemass act 1 - Fire

I’m having an issue with the boss Candlemass in Act 1. I don’t know if it’s a bug or something else. When he uses his fire attack, sometimes I can’t see the fire coming out of his mouth, but the attack still works (even though I don’t see it).

This has already happened to me twice in Hardcore, and it killed me both times. I don’t understand why I can’t see the fire sometimes, and then I get killed in a second without being able to react.

Why is happening this?