When riding a donkey or mule with a chest and a sadle the donkey or mule looses half of it's health

When riding a donkey or mule with a chest and a saddle the donkey or mule looses half of it's health

*What I expected to happen was
The donkey or mule has full health.

*What actually happened was
The donkey or mule lost half of it's health.

Steps to Reproduce:
1. Put chest and saddle on donkey or mule.
2. Wait for about 5 minutes.
3. Mount the donkey or mule.