Some times when you switch from one unit to another the previous unit can become invincible to bullets shot in his back.
You can still kill him by shooting him in the legs, he also won't turn around to return fire but this may be due to the old A2/OA bug where they become dumb.
{F21826}
I have two units on the map one player and the other playable.
They're set facing away from each other, when the mission starts I do nothing but press U the switch to the other unit.
If I now turn around to face the previous player unit and shoot him in the back he's usually invincible to bullets.
I've also noticed that if I remove the ammo from the target using the slider the unit seems to die in the normal way.
Unable to reproduce in version 1.10.114486.
Could you please try in this version? Thank you.