I think I might have a bug report! I assaulted a landed small UFO, and the first turn I saw a muton facing away from the Skyranger right next to the open cockpit. I sent one of my attack dogs it in to attack it in the back and draw the muton's reaction fire. But the muton never reacted to any of the 5+ bite attacks, even up to it's death.
I save scummed and confirmed it did turn around when troops shot at it, meaning it did have time units for reaction fire. But the enemy didn't register the dog's attacks as something it would fire against. This doesn't seem like it should be doing that.