Untraversable Stair Bug

I sent a soldier up to get rid of a pesky harridan and couldn't get him back down the stairs for some reason. There was nothing blocking the bottom of the stairs, the game just didn't accept any commands for moving to the lower level.

Eventually I got it to work by getting another soldier to stand at the base of the stairs and then moving him away again.

Also a question: Is it possible for soldiers to drop weapons due to enemy fire? The previous mission I was about to storm a corvette before I realized one of my best soldiers was missing her laser rifle which was in fact halfway across the map.