So on the latest snapshot on hard difficulty no skeletons zombies spiders or creepers spawn in my world at night or in caves. Have been playing for about 20 mine craft days. Have tried creating a new world and setting time to night and have seen about 1 creeper and 2 spiders spawn after a while of searching at night time. But no matter what i do in my actual world created in the latest snapshot 19w08b no skeletons zombies spiders or creepers spawn ever, everything else is fine.