I have not actually played with this, but as I was looking through the source, I saw that the offset function actually offsets all the directions by the X offset. I'd just like to let you know while I'm looking here.

public void offset(double xOffset, double yOffset, double zOffset){
	this.x += xOffset;
	this.y += xOffset;
	this.z += xOffset;
}

Dang, nobody has ever gotten that deep into the code. That class was one I made in anticipation in moving away from Minecraft's Vec3D class, and isn't currently used anywhere, so I probably should just delete it. Will fix.

Corrected in V15.9.0.