Generating a flatworld of 1 block thick makes you fall through floor on first spawn

I found this bug where you fall through the floor into the void on first spawn when you generate a flatworld which is only 1 block high.
My Presets: "1x155" Settings: Bonus Chest: False | Allow Cheats: True | Gamemode: Creative
How to reproduce:
Generate a flatworld (maybe on creative) and set the presets to "1x155"