When I create world with following preset string, minecraft gets stuck and uses lots of CPU.

 minecraft:bedrock,254*minecraft:dirt,minecraft:bedrock;minecraft:plains,decoration,mineshaft,lake,lava_lake

The world also does not behave as expected. I was expecting to be inside stone completely, with preference of prefab natural tunnel, but I was spawned on top of the topmost layer, and villagers stuck on sky. I had to kill the java process to be able get out of the stuck game.

Thank you for looking into this.