She is a gal who uses a weapon with infinite ammo, yes (Smasher: STR_LASER_PISTOL), which Piratez has modified into some kind of laser shotgun. It is my first deployment of these guns, so maybe that's it?
I do use other weapons with unlimited ammo though (Zappers: STR_ZAP_GUN for instance), and plenty of melee weapons, none of which ever gave me issues.
I'm just about to start testing with removing her feats, I'll start with those that are related to that weapon.
Edit: I just looked into the save and she has a "kill" with:
weapon: ""
weaponAmmo: ""
No idea how she got that, but I'll keep an eye open for it in the future. Maybe something like stunning someone who was standing in a fire, who then died from the fire? I think OXCE handles stunned units differently from vanilla, maybe that's it?
Edit2: Indeed, filling up those fields (I just put STR_SHIELD_AUX) made everything work again. The gal got the award for killing people using stun damage, so it does seem like it's one of those weird situations. Maybe it's not good to let people burn after they passed out..
Note: the victim would have been killed by environmental fire while stunned. I first threw a molotov as I was approaching, then stunned it on the next turn while it was standing in the fire (can't tell if it was on fire, that's hard to see for units standing in fire) and left the stunned unit there in the fire.