This problem is driving me insane guys.
For some reason my characters eventually disappears when my camera goes out of view of it.
Sometimes walking up to the character will make it reappear again.
This video shows a quest NPC that has vanish. Its still though and the game object is active you will see me complete a quest. Then you will see me go to another NPC that is also disappeared. I make it show again buy disabling the game Object and then re enabling it.