I am in the 14w19a snapshot and building a multi-level mob spawner. As I was finishing up, I got inside the spawner and started to take out torches. As I went through the spawner I noticed that there were no hostile mobs spawning. I double checked that my difficulty was on hard and continued on. I encountered one creeper during the entire project and that was it. I do not run any mods and I am sure I'm on hard. I don't know if this is a bug or a new fix.