Mobs are not displayed when viewed at an angle of 45 and 90 degrees. To repeat this bug you need:
1. Create a world of any type.
2. Climb to the height of y = 63.
3. Build a "box" of blocks (for convenience) at this height.
4. Go inside this box.
5. Spawn Ghast.
6. Become near Ghast.
7. Look at the Ghast at an angle of 45 degrees. As you can see, he disappeared.
8. Look at the Ghast at an angle of 90 degrees. As you can see, he also disappeared.
I downloaded an archived map on which I saw this bug. When you start the maps, you will spawned in the box, so follow steps 6, 7, 8.