I created a survival world and I had it on hard and a lot of hostile mobs were spawning. After a while of playing I locked the difficulty to hard and now hostile mobs barely spawn at all. I've tried all sorts of thing like changing the difficulty, using NBT and turning off the locked mechanism but nothing's worked. I also had originally made this world in 18w48a and, to my knowledge, that was a pretty glitchy snapshot, so that may have something to do with it, but it hasn't changed since I updated to version 18w48b.