This is intended. Pokémon Vortex is based on (Pokémon) main games, but many things and mechanics work different. One of these differences is what you reported: mega drain (giga drain, dream eater, etc.) doesn't have the side effect of healing the user.
Attacks such Self-destruction or Explosion didn't take user's HP, also!
Moves like Rain dance, Sunny day, Sandstorm, Hail, etc... don't exist (at the moment of this post) in Vortex
Z-Moves don't exist in this game, also!
But, we have pokéBay, Vortex Store, 6 variants of each poke, cool events too!!!