Mobs don't spawn in dark places.

I think this works as intended or the bug affects all monster spawning rates and we should investigate further to generalize the problem.

I tried as you suggested and can confirm I didn't get any monsters. Then I did the same in a new survival world (difficulty on, dove down a dark cave for some torchless spelunking) and also did not get any monsters spawning.

To do my torchless spelunking in both attempts I cranked the game brightness to max and the phone brightness to max so I could see whether or not I was going down the cave or stuck in a shallow hole.

Samsung Galaxy S4 Android 5.0.1 MCPE 0.11.1 (version of 0.11.1 that updated June 11, 2015)

This works as intended; you'd only get spawning in a radius of 16 blocks away from you and in a cave you don't have that radius. To further add on to that point, at nighttime mob spawning in caves is severely maimed due to the abundance of spawn spaces on the surface.