Was testing the custom terrain, set all sliders on page 3 as far left as possible. Heard pigs, didn't see any. Dug away the gravel where the sound was and revealed spawned pigs. Once they climbed up to 256 I could again hear them, but not see them.
Of note is that they were not suffocating in the gravel, and could not be seen even with Spectator mode's no-clipping while in the gravel.
Is this still a concern in the current Minecraft version 14w30c or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.