What you were expecting to happen, and what actually happened:
When casting Vash’Dagon, he devours then summons, then deals damage. However, I am repeatedly finding his summon blocked by my Sorcerer’s Spawn of Hell talent, which gives a 50% chance to summon a Hellspawn when an enemy dies. Surely Vash should summon before damage is dealt?
How often does this happen? When did it begin happening?
All the time