The behavior of stun gas IS very odd, I won't deny that. But brainsuckers are FAST. When a brainsucker runs through 8 spaces of stun gas and attacks your soldier, it's your fault you didn't have proximity mines or anyone ready with reaction fire. Simply having the reaction fire available seems to screw with the AI, causing brainsuckers to spin in circles until they pass out. If you were that close to a pod in turn based mode, you were close enough to run over and pick them up. If this is not true, you are pushing too hard and too fast and you got punished for it. It's not the game's fault you weren't prepared. Brainsucker pods have long range proximity sensors and will not hatch if targets are out of range. I don't know if that includes civilians or aliens, but it is possible to deny hatching if your guys aren't there.