Background:

The most straightforward method of transporting a Shulker mob to the Overworld is to send it through a gateway to the main End island, then from there through the Fountain to the Overworld. I have personally used this method in previous versions, and there are recordings of others doing so on previous versions (a good example may be found at https://youtu.be/1A_MJbYNeMs?t=878).

Summary:

Shulker mobs disappear when sent through an End gateway.

Steps to Reproduce:

1. Create a new world in Creative mode.
2. Build an End portal and fill it with Eyes of Ender to open the portal and travel to the End.
3. Execute /kill @e [type=ender_dragon] to kill the Ender Dragon.
4. Using an Ender Pearl, travel through the outer islands gateway that is produced on the main island.
5. Once on the other side, build a short line of blocks leading out from the gateway, level with the bedrock.
6. Place powered rails leading up to the gateway portal. Power the rails.
7. Place a mine cart on the powered rails.
8. Using a spawn egg, spawn a Shulker mob on a block immediately adjacent to the mine cart. The Shulker mob should immediately enter the mine cart.
9. Press the minecart towards the gateway portal. Ensure that the mine cart is pressed as far against the portal as possible.
10. Break the mine cart. The Shulker mob will immediately go through the portal. This is confirmed by the gateway's shooting out a vertical beam of light.
11. Using an Ender pearl, travel through the gateway back to the main End island.
12. Execute the command /testfor @e [type=shulker]. The game will return the message, "No targets matched selector." The Shulker mob has disappeared or has been sent to the wrong coordinates.

The steps outlined above are a relatively quick way to reproduce the bug. You will achieve the same result regardless of which gateway you send a Shulker mob through, and you will achieve the same result regardless of whether you send a naturally generated Shulker mob or an artificially spawned Shulker mob through the gateway. I have reproduced this bug on Minecraft for Windows 10 and on Minecraft for Nintendo Switch (video of me reproducing the bug on Windows 10 here).

I do not think that this is the same bug as MCPE-157125, which warps Shulkers to Y-levels of 500+ when sending them through the Fountain to the Overworld. I have personally tested as high as Y = 33000 in the End after sending a Shulker through a gateway and have not found any Shulkers warped to the wrong Y-level yet.