Mobs Not Spawning in Newly Created Areas

The latest update (now version 1.2.10) stated that it corrected the bug of creatures not spawning in new areas. I wasn't aware of the bug until I realized that my 11 level/room mob grinder that I carved from nearly bedrock to surface was not producing even a single creature. Most rooms are 18 x 18, and of course totally dark. But to be absolutely certain I expanded one room to twice that size and not one creature has ever spawned. On my old Xbox 360 account, they would spawn very well, so I know it should work. Must I create a new world for the update to take affect?9
[Update: version 1.2.10 I have now seen a few creatures in my mob grinder, but the spawn rules are still quite broken. After 15 minutes, only 4 creatures spawned.]