I just had a challenge "Kill 4 radroaches without weapon", and it does not work at all.

I tried removing the weapons from dwellers during an attack and none of the kills was taken into account.

I even sent a dweller without weapon in the wasteland, already he killed two radroaches, but the counter does not move either.

It seems this challenge is completely bugged.

The dweller in the wasteland might not count until it returns. The others may not count if they had weapons at the beginning of the attack.