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 rifle, 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.
Best Answer
If an enemy is killed while under mind control, you get their equipment.
Whether this is a bug or by design, we don't know, but it is consistent. Maybe the idea is that the MCed muton calmly laid down its arms before standing up in front of the sectopod? :-)