Mobs created by spawner never despawn

I've been playing on my friends realm. I went afk at a mob grinder I recently made, but the trident killers werent working, and I had a load of mobs and a laggy mess to deal with. 

I decided to move away from the farm to despawn them, And I came back 5 minutes later to find that no mobs have despawned.  And I had to clear them out myself.  :/