While trying to get the "Feels like home" advancement/achievement sending a strider through a nether portal into an Ancient City the strider vanishes from existence. 
After travelling through the portal after the strider the strider is non existent on the other side. 

After experimenting this seems to be the case with any mob.

To recreate: 
1) Create Nether portal in the ancient city
2) Travel to Nether
3) Lead mob to the portal (Strider in this case) 
4) Mob travels through portal
5) Travel through portal to Overworld

Expected Result: Mob appears in overworld 
Seems to effect any mob whether passive, friendly or hostile, not limited to Strider