In previous versions, using caves generation for buffet, where lava lakes would be in the nether were replaced with water lakes. Also, there would be no netherrack in the overworld just the biome appropriate blocks.

Steps

• Create New World
• More World Options...
• World Type: Caves
• Customize
• Biome: Plains

Expected

• Large lakes at y=32 would be water
• "underground" blocks would be stone variants, dirt, gravel, etc (based on biome selected)

Actual

• Large lakes at y=32 would be water
• "underground" blocks are all netherrack