When I pushed F5 twice and looked up the world below me should have rendered

Instead the chucnk below me and other chunks below it didn't render

1.Click F5 twice, (quite hard)
2. then look up (even harder)
3.Chunk below has gone bye bye

This is probably due to the new optimizations where only chucks which are in sight are rendered, the game must think you are looking up so there is no need for rendering below the player/ "out of sight" (also explains extremely high frame rates when looking up not that matters). Also you can be in survival but you can get a better view in creative. Chunks behind the player also don't render as well, I know that is intentional but the game doesn't know that you are in another view and renders the same way may not be.