Spawn at Level Y5 when creating custom super flat world

Expected Result
Spawn at Level Y64 above the blocks.
 
Actual Result
Spawned at Level Y5 inside the blocks.
 
Steps to Reproduce:
1. Generate Super Flat world with the preset:

minecraft:bedrock,63*red_concrete;1

2. Click `Create New World`.

3. Witness absolute darkness, as you spawn inside the blocks at Level Y5.