I had a mob farm that after updating to the snapshot 18w43c randomly wont spawn mobs, I last used it in 1.13.2 so it could be any of the snapshots. It is in a Cold Ocean Biome but used to work fine until I recently used it after updating and nothing spawned and the entity count kept saying 0/1, I haven't changed anything on where the mobs spawn so I assume it is a snapshot bug but I don't know what is causing it. The mob farm's spawning area is mainly cobblestone and is 2 blocks tall of space for each floor.