Doesn’t seem to have been reported before, so hopefully it’s not a repost. Tl;dr here’s what happened:
FK fell into into the floor at the very start of the map, as if he were submerged in the ground texture, but managed to get out. When we found the chest, it was submerged in a similar fashion. As we approached it, we fell through the floor and got stuck there, beneath the chest. We were hopping around trying to figure a way out when I decided to make a small recording. Eventually I had to kill the FK and then jumped around until a little rat managed to poke me to death.