Maybe relates to:
The bug
When you are in the void and press F5 once, you can see an additional black square. I don't know if this is intended, but I don't see any use for this.
How to reproduce
Press F5 once
Start flying in creative mode
Use this command:
/tp ~ -64 ~ 90 -50
I guess this is the graphic which creates the effect that when you are looking up while falling in the void, you will only see a white square, but it is anyways pretty bugged
Confirmed for 1.13.1.