I don't know if this should beclassed as a bug or not, but If I have a melee tank and a range unit, then I would expect damage to be dealt to the tank first.

Example 1: My forces, 2 tanks + archers.  Enemies: all melee units.  Result, my archers took 34 damage, and the tanks took 0, I win, but the cost was too high.

Ran the same battle through tactical with me controllong, archers took no damage, and the tanks took a small amount of damage each (< 10).

I think there are some battles that appear to swing the other way too.  Champion with 27 hitpoints, armed only with dagger, no armour.  Attacked 3 stacks of 3 mites.  their base damage is 9, hitpoints at 15hp each. 

Result of auto resolve (outside of tactical), Champion took 9 damage, and killed all of them.  I had trouble using my Sov in tactical battle very similar to this, and he was armed with a sword and had regeneration on!!!