So I was in a cave when I spotted an Enderman. When I hit it, no red border showed around the mob (the red border is the one seen around the baby horse in the photo attached). Spawning and hitting a zombie in the cave confirmed the problem.

Steps to Reproduce:
1. Go into a dark, low-elevation cave
2. Drink a night vision potion
3. Spawn in any hostile mob. Hit it, and see the problem for yourself.