Mobs Not Despawning

Hostile Mobs don't despawn naturally. I was AFK and around 20-30 mobs spawned around me. Other players noticed that they were getting no mobspawn. One of the others Tp'd to me and found This 

I came back and went into GM3 as the only person on the server. Mobs did not despawn. Relogging also did not cause them to despawn. Again, I am the only person on the server. I have attached a small clip to show what I mean