Escaping Animals When i Go to the Nether

What I expected to happen was...:
What i expected to happen is my animals will not escape when i go to the nether

What actually happened was...:
When i was going to the nether to mine quartz i went back finding my animals escaped for no reason(even if i surround them with fence though)

Steps to Reproduce:
1. Place Animals And Surround Them With Fence
2. Go to the Nether
3. Come Back From the Nether You will find your animals outside the fence