Summary:-

The overworld build height is only 256 blocks rather than 320 which was added in the new Java snapshot 21w06a. 

How To Reproduce:-

1:- Create a new world

2:- Grab any block and build up to the build height in the overworld. 

Observed Results:-

The build height message appears saying 256 is the build height.

Expected Results:-

The new 320 build height should appear and not 256.