Considering the undead race are the only ones who could use the spell this seems like a bug. I am playing on my Samsung Android S9 and bought it with expansions for nostalgia.
Drain Health works fine you take the enemies health for your own; Mass Drain Health though it hits the enemy and by the time it touches the player character it results in instant game over. I rolled an Undead Priest because I knew that meant the health drain spells but this one doesn't work right.
I think it's a bug so I thought this belonged here but not sure if it would be fixed.