While inside of a tree (or possibly any other similar space) the rest of the world unloads to prevent lag. This is fine unless fancy graphics is turned on.
Environment
Minecraft Version: 18w43c
Operating System: Windows 10 (amd64) version 10.0
Java Version: 1.8.0_51, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle
I can partially confirm this - tested and everything outside the chunk I was in was unloaded when I looked down greater than 40 degrees. However, at other times everything loaded as you would expect.
Edit: after reloading the world, it happens when I look at any angle.