Some Spawned In Mobs Randomly Freeze and Are Invincible

While roaming caves, I found every type of mob that I could find randomly frozen in caves. They seemed to show up, but they didn't move, attack and are not able to be hit. In the image shown, there was three mobs frozen in the same cluster, and I was staring directly at the enderman. I'm sure this is a bug that should be fixed.