When you're using any maps to load terrain, it loads the terrain map around it, however it doesn't if the terrain is located below y=0. Instead of anything that is below y=0, it shows black square as if there is a void below, when it is not. This only happens by setting Caves & Cliffs experimental toggle on in the world.
Step to Reproduce
1. Do /fill ~ ~ ~ ~10 -10 ~10 air
2. Get a map and load it up.
The map will load properly, even if the terrain is below y=0.
The map shows a black square if the terrain is below y=0.