This unrevolved issue 4 years ago was posted on MCPE-46932, but that ticket failed to mention how to properly reproduce, or solve the problem. If you do 

1. /spreadplayers 0 0 0 1000 @
2. It will teleport you to the sky at y=319 IF THE CHUNK IS UNLOADED 
3. Additionally, it will occasionally tp you over the ocean as it fails to detect that surface level is water.

Clearly to resolve this issue the program must load the chunk first, or find the correct y-value before tping you to that location.