Edit: Was caused by using Optifine.

After flying away from mobs in a an area that I want to keep I came back to find them completely invisible, most of them were named while some of them held items to keep them from despawning, upon closer inspection I found that they still existed and were able to hit me but were still completely invisible, unable to be seen or hit even when contained in a 1x1 area the mobs were zombies.

What I expected to happen was...:
Mobs should load correctly after being in unloaded chunks

What actually happened was...:
Mobs completely vanished but were still there when minecraft is reloaded.

Steps to Reproduce:
1. Name, or otherwise get a zombie, skeleton, or other hostile mob (may affect others haven't tested it) somewhere.
2. Get the chunk they are in outside render distance, in my world it took 2 tries with the second one going about 500 block away.
3. Come back to where you left the mob.

4. Reload the world and the mob should be there again