Leads Disappearing When Mobs are Tied Up

When I tie my horse, donkey, and mule to a fence using a lead, if I ever leave them and go far away, when I return a while later the leads are gone. The mobs are there and move freely, as if the I never leashed them.

Steps to Reproduce:
1. Log onto minecraft
2. Lead a mob, horse, and/or donkey to a fence
3. Walk very far away, and about half an hour later return and the leads should be gone