Hostile mobs spawn significantly less when a player has moved far from the spawn in SMP.

I was exploring far from the spawn point - also the point where everyone on the server lives. When I returned to the spawn I noticed hostile mobs weren't spawning in caves or at night (however mob spawner blocks still worked). The bug was fixed by rebooting the server. The bug happened again when after traveling in the nether I made a portal and exited, even further from the spawn the last time. Upon returning I discovered that again, hostile mobs have stopped spawning. In both cases I was the only player on the server at the time the bug was triggered. The bug does not effect the spawning of hostile mobs.

What I expected to happen was:
When I travelled far from the spawn and back that hostile mobs would still spawn naturally.

What actually happened was:
Hostile mobs stopped spawning on the server.

Steps to reproduce:
1. Play SMP alone
2. Travel a large distance from the spawn, some 500+ blocks (rough estimate)
3. Return home
4. Hostiles mobs should now stop spawning when they should.