I generated a new world in VanillaCubic and there was a chunk error as the world was generating.
https://www.dropbox.com/s/hxfq211q323kv3h/2017-05-25_19.42.07.png?dl=0

This is "just" one chunk generating with wrong seed/position given to the generator.

The only explanation I can find is that this is vanilla bug, caused by concurrency issue when cliet attempts to generate it's own biomes before server sends any chunks.

I'm almost sure this is vanilla booms generation bug causing it.. Closing for now.. Comment here if it happens again.