I managed to grab a screenshot, but unfortunately, using the in-game screenshot feature froze my game. So I had to rely on my party members to get the screenshot showing why they were able to become invulnerable. I have no idea if the Legendary Defender is intended to be there, but it allows people to become invulnerable and take the supply camp when everybody decides to run off, since they wont be able to kill them.
What happened was, we were zerging the supply camp and took it. There was an enemy zerg there, that we managed to kill and the survivors ran to the back of this supply camp and were able to sit on those ledges whilst invulnerable.
The camp is the refuge in the south-east corner of the three battlegrounds.
