I saw the issue become "resolved" during the 1.13  snapshot releases. However, this issue has come up again and I'm not sure how it happened. All I know is that my  only one of my worlds has this issue.  I did find a witch hut earlier with a witch and right before i came to report this i had seen a skeleton and spider but it had been a couple of in game nights since my start to actually see random mobs spawn. Also note that i had seen the mobs after i used the "optimize" button for the world.