Soul Cage is fairly unreliable when it comes to the amount of mobs it spawns each time. A Tier 5 shard will spawn up to 5 instead of 5.

This is not intended behavior. Looking into what causes this.

Initial fix attempts have been... subpar

http://gfycat.com/HealthyCoolFoal

I think I'm just going to consider this a side-effect of changed logic in 1.9. It only affects you if the cage is directly on the ground.