I just noticed a small timeing issue in the loss of mana after get hit by a mana draining hero.
I was playing raid and one of my heros (Boril) got full mana on my switch.
Also this swich fullfiled the mana of an enemy hero…well i dont remember which hero was it, AOE hit and mana drain. He casted it hit all my heroes got hit by it (including Boril he got damaged).
During the attack animation, just at the moment when my turn next turn started and the mana loss take place, I was tapping Boril hoplessly. (This happend almost at the same time)
With this i was able to cast his special skill before he actually lost the mana.