Flatworld Generation Bug

I have encountered a bug where normal worldgen cuts into my flat world. this also includes the old lava and water update bugs. I have used the default flat world settings, default game mode set to creative, and the name was "Instant world". Pictures attached. I have allowed cheats.
I believe that this bug is biome specific with a rare biome, as I was unable to duplicate the bug except on the same seedcode: 5733684037147482083
I know that I am supposed to see flat world generation, not the generation I am seeing.