Confirmed on iPod 5 (iOS 8.3) running 0.11.1. This relates to MCPE-188 in the way that it has to do with underwater-viewed blocks.
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.
Confirmed on iPod 5 (iOS 8.3) running 0.11.1.
Confirmed on iPod 5 (iOS 8.3) running 0.11.1. A workaround is to change your controls out of split touch, restart, and change them back. This has to be repeated every restart/launch, however.
Confirmed on iPod 5 (iOS 8.3) running 0.11.1.
Confirmed on iPod 5 (iOS 8.1.3) running 0.11.1.
Confirmed on iPod 5 (iOS 8.3) and iPod 4 (iOS 6.1.3) both running 0.11.1.
Confirmed on iPod 5 running 0.11.1a.
They probably spawn the same amount, it's just that since there are more places for them to spawn, they spawn farther apart.
You are supposed to break it with shears to get it back, I believe.
Have you lighted up areas of the map? Or are there dark spots underground?
Confirmed to be fixed in 0.12.1. This can be closed.