If you push a shulker into the end portal with a piston, the shulker dissapears but when you go through the portal yourself, the shulker is not in the overworld.(How to recreate - see attached images before_push and after_push). I tested dropping items and entities into the portal and they appeared fine as usual in the overworld. I placed the shulker with a spawn egg in the portal block itself (image placed_in_portal, I managed to screenshot in the one tick before the shulker went through) and it appeared in the overworld just fine, so the problem is somehow related to the piston pushing-in part. When dropping the shulker into the portal in a minecart, neither travel through, they just sit there (image in_minecart), and so I currently do not know how to get a shulker into the overworld while these bugs exist. I also tried to push the shulker in while named with a nametag to test if it was somehow a despawning issue but it didn't make a difference.

EDIT: I have done further testing and I found that the shulkers to teleport to the overworld, the actually appear at 0,0 regardless of the spawnpoint or spawn chunks