mobs are immediately transfered back to the overworld upon spawning in the nether

I was in single player holding a carrot and a pig was following me. I wanted to take the pig to the nether for a nether farm. I walked through the portal and the pig followed me untill he touched the portal and got transfered to the nether. I walked into the portal again and got tranfered to the nether too. the moment I spawned in the nether portal the pig was standing next to me in the portal. after 1 seccond the pig got transfered back to the overworld. other

What I expected to happen: the moment I spawned in the nether the pig should walk out of the portal.

what actually happened: the moment I spawned in the nether the pig got transfered back to the overworld.

steps to reproduce:
1. have a mob (any) walk into a nether portal.
2. when it has transfered walk into the portal yourself and wait untill you are transfered yourself.
3. the moment you spawn in the other portal the other mob will be tranfered back to the overworld.

Adittional information: this ONLY happens when a mob goes to the nether. not when it goes from the nether to the overworld