Height limit:The Flat World with Experiment:'Caves&Cliffs'

As we know,some of the rest part of 'Caves&Cliffs' Update has been put in Experiments.The height limit of the world should be widen with Experiment:'Caves&Cliffs'.

In fact,it is only true for World Type:Infinite,but it's ineffective to 'Flat'.

Days ago,before I wanted to start to build ****************,I tried to put blocks under zero,but a message came and meant that the lower limit is y=0（The world type：Flat）

 

So,Is it contradictory between the fact and the tidings of the Update Plans,isn't it?

To sum up , it is possibly a bug to 'Flat' that's what the update probably forgot or left out.