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.
MC-138208
While inside a box of transparent blocks, the world unloads
Resolved
Fixed
3
(Unassigned)
Confirmed
graphics, rendering, unloaded-chunks
Minecraft 18w43c
Minecraft 19w09a
Retrieved
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.