Platform, device version and operating system:
What were you expecting to happen, and what actually happened:
Black Beast is having a hidden dispel ability and it heals back to full health, Vash’Dagon is not having one neither does he heal. Ultra-rare vs. Mythic irony again.
I strongly suspect that you missed adding the dispel ability on Vash’Dagon. Black Beast has it, but it’s not listed in the description, only in the spell description of effects.