Freeciv - Whole Stack Destroyed by Single Attack

I find that in Freeciv, when you attack a whole stack of units (irrespective of how large this stack is), if your attacking unit is able to take out one of the strongest units in the stack, the entire stack gets destroyed. I don't understand how this is explained by anything in the CombatRules.

Haven't other people encountered this problem/anomaly? Is this a bug or feature?