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).
Shulker mobs disappear when sent through an End gateway.
Steps to Reproduce:
- Create a new world in Creative mode.
- Build an End portal and fill it with Eyes of Ender to open the portal and travel to the End.
- Execute /kill @e [type=ender_dragon] to kill the Ender Dragon.
- Using an Ender Pearl, travel through the outer islands gateway that is produced on the main island.
- Once on the other side, build a short line of blocks leading out from the gateway, level with the bedrock.
- Place powered rails leading up to the gateway portal. Power the rails.
- Place a mine cart on the powered rails.
- 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.
- Press the minecart towards the gateway portal. Ensure that the mine cart is pressed as far against the portal as possible.
- 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.
- Using an Ender pearl, travel through the gateway back to the main End island.
- 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.