Entities transfer from the nether using the portal despawns when going back to the overworld.

Help! I just got a wither skeleton and I pushed to the portal and it doesn't transfer it!

To find bug:
1. Go to nether and spawn a mob next to the portal.
2. Push the mob to the portal and go back to the overworld.

Bug found!

NOTE: IT DOESN'T ALWAYS HAPPENS!