Bug in floating island world generation

Hello! I decided to try out some of the new world generation options - I made a floating islands world set to the Plains biome. When the world loaded, it seemed fine at first - there were grass blocks everywhere. But where it seems like stone should have been, there were end stone blocks instead. Again, this was set to the Plains biome on the Floating Islands setting. The game version was the latest snapshot, 20w21a.