https://pastebin.com/Vn9JSr1S (ChunkData.setRegion() located at line 147)
this generates an empty void world, when it's supposed to generate a flat world.

if result.setRegion is replaced with 

for (int X = 0; X < 16; X++)
{
    for (int Z = 0; Z < 16; Z++)
    {
        result.setBlock(X, y, Z, materials[y]);
    }
}

it generates the flat world properly.

plugin attached generates an empty void world while it should be generating a normal flat world.

ChunkData.setRegion() worked properly in 1.12.