Leads tying animals to fence disappear

I have tied llamas to a fencepost with leads. This was fine for many days. When one baby llama grew up, all the leads disappeared. The animals wandered off as they were no longer tied.

What I expected to happen was...:
The leads should not disappear. The animals should remain tied to the fence.

What actually happened was...:
The leads disappeared.

Steps to Reproduce:
1. Tie llamas to fence
2. Ensure one of them is a baby llama
3. Wait for Baby llama to grow up

4. Leads disappear