Version Information

Minecraft version: 1.12.2
Forge version: 1.12.2-forge-14.23.5.2857
Other mods (if applicable):

FastAsyncWorldEdit

Steps to reproduce:

1. //wand
2. Select An Area In A Super Flat World
3. Modify A Large Portion Of Terrain To Suit Build Needs, Happens From Creating New Terrain, Typically Above The World's Surface Y Level.
4. Tunnel Within Terrain, You Will Find Sunlight Within The New Underground Sections

Expected Behavior:

No Sunlight Inside New Terrain. Makes Replicating Underground Real World Structures More Time Consuming, You Have To Continuously Rebuild The Chunk Until It Goes Away, Even Them Sometimes Coming Back

Actual Behavior:

Random Cubic Chunk Sized Patches Of Sunlight Will Appear Inside The New Terrain. Cannot Be Resolved Without Deleting Everything Within The Space Then Redoing The Terrain Until It Works, Very Very Time Consuming To Fix Lighting Bugs Within Terrain. Especially When They Appear After You Have Built Within The Terrain.

Screenshots of the issue have been linked below. Please resolve this bug; It has been making building with real world topography values quite tedious for my builders & myself.

Link to crash log (if applicable - use a site such as hastebin: