Just as the title says. My scenario was:
I had been roaming the End Cities when I for some reason felt very tall in-game. I went up to an Enderman for a quick size comparison and looked up at his head, when he went invisible. The particles actively produced for a few minutes, and when I looked down he was standing there (Meaning he hadn't teleported).
So, I checked around with it and found this: If an Enderman's legs/lower body is out of your FOV, then the Enderman will turn invisible to the client player. This is similar to older bugs with other entities (minecarts, etc). Screenshots are included (Please note in the screenshots my head position changed slightly).