They just reached the edge of the map area, which technically due to how it's implemented will be black like that. True that it probably shouldn't normally be exposed like that, though know that it can happen if someone digs it out, some explosive went off near the map edge, or there was a fight that destroyed the walls there.
This particular case happens to be something that can definitely be addressed, though. Ideally I'd need the map seed to be able to test this one--it's caused by a a prefab that in this particular layout managed to dig itself right up to the map edge.
I'd need the map seed to be able to test this one--it's caused by a certain type of prefab that digs into the walls.
Any way to get the seed from this run? I don't see this player on the leaderboards so they might be using a different name.