Superflat world build height is only 256 blocks

Summary:-

Saw ibxtoycat's vid and wanted to test it out to check if the build height is actually 256 or 320. Turns out it's 256.

How To Reproduce:-

 

   1:- Create a superflat world.

   2:- Once entered, tower up to the build limit.

 

Observed Results:-

The build height says 256. 

 

Expected Results:-

It should have the new 320 build height.

 

Note*

I don't know if this is intended or not.