I found bugs in the custom world generation.  I think that the bug isn't fixed yet. I created a floating island world and there was still endstone, and yes, i am sure I was in the newest pre release. I created it again and there was no land, just structures. In the cave world everything was right until I choose a biome. There was netherrack and lava. And I think the worlds that you didn't choose a biome should generate with every biome, but it's generating olny in ocean