First, let me create a scenario.
I've got a soldier (no specific soldier) with psychic ability
mind control. he is facing two enemies, namely a muton and a sectopod. I've used
mind control on the muton and succeded. I ended the round and as usual, the muton is flanked by the sectopod and killed the muton. I've killed the sectopod and the round finishes, thus winning it. At the rewards I've received a
plasma pistol, and a
alien grenade along with the 6 weapon fragments (yup, I've calculated it).
So, is this a bug? Take note that I didn't even bring an arc thrower in this mission because I've interrogated every alien at this point.