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So I was just chillin' in the Nether, collecting Nether Quartz, and suddenly a Ghast takes shots at me. As I fought it, I noticed something different about them. Not their appearance, but...a small thing that was nagging at me.
Then I realised that whenever the Ghast went off the screen, it...how do I put this? Let's say when it went off my screen, before it glided off, it suddenly disappeared quickly. When I turned around, it came back. When a Ghast exits the screen it disappears before it is fully off the screen.

If you don't understand, locate a Ghast and look at it. Then turn away slowly, but keep an eye on the Ghast, and you'll see what I mean by this.

This is possibly to do with the update.

It probably has to do with rendering. My guess is that they've coded the game to not load the graphics for any entities that aren't in your field of view. Since the hitbox of a ghast is a lot smaller than the model, it would make sense that it happens when the ghast is close to the edge of the screen.

I once had to get inside a Ghast to kill it.
In Creative Mode, of course.
But that is the only conclusion I came to. It doesn't seem to happen with any other entities, and the Ghast still technically moves around outside my view.

Rendering has nothing to do with any behavior of any entity - it's simply drawing the graphics. The ghast can still move around even when off-screen because the chunk it's within is loaded.