Caves filled with lava below y=-64

When making a world with a custom height, all caves would appear to be filled with lava if the world is deeper than level -64. For further explanation, when I make a world that is -256 deep, they appear to be flooded with lava from -64 all the way to -251. In an uncustomized world, they all appear to be flooded with lava at level -54 instead of -57.