After throwing a stun grenade at a group of enemies, they are stunned for two rounds (according to mouse-over info), and they are unable to take any action on their immediately following turn. At that point, the mouse-over indicator changes to show they are stunned for one more round, but they can take their normal action on the following turn.

So despite clearly indicating the enemies are stunned for two rounds, they only miss ONE turn. If one of my NPCs is also stunned in the blast, that NPC is out of action for TWO turns.

In version 0.485, the enemies would lose two turns of action when stunned - although I would point out that while stunned, an enemy was immune to subsequent stun grenades, so you couldn't wait a turn and throw another stun grenade and set their stun level back to two. I don't know if that was intentional or not, but if it was, it seems odd that there's a mechanic specifically designed to prevent the player from pinning down an enemy. If it wasn't intentional, then it would be nice to fix it so that a stunned enemy is susceptible to a stun grenade.