Mobs will spawn while playing for about 5-10 minutes, then spawn very rarely. Easy level may go several game days without a mob. Switched to Normal, got three zombies total in a night, nothing else. Went running through mountains and mesas looking for mobs to kill, nothing.
Started with the most recent update.
If I exit the world, then return to it (not exiting game), the regular spawn rate will return for a few minutes.
Sure, it makes it easy to mine all kinds of swag, but it also slows XP and nerfs the game toward boredom.
Take a look at MC-42174 which affects spawn rates on the newer versions. Or, in reality, it causes zombies to not despawn and thus you can wind up with pockets of them underground somewhere, all holding chicken eggs, hitting the mob cap, and thus stopping any new spawns.