I can't reset my Realms world

I was configuring my Realm, and I wanted to replace the puzzle map "Tranquil Gardens" with one of my own worlds. A close friend of mine can't use LAN, so I was asked to upload our world to Realms so she can still play.
The game should have loaded my world, given me the Done button, and then when I enter the world, it's the one I uploaded.
But what actually happened was that the game said it was loading my world, and then when the loading bar reached 100%, it gave me a button that said "Done" with a semi-transparent "Cancel" text behind it. I click the button, and surprise! The Tranquil Gardens icon is still showing up!

And this isn't just an issue with the icon. When I click the image, it tells me I need a custom texture pack to play on the world, implying that no, my world has NOT been uploaded for some reason!

Steps to Reproduce:
1. Have a realm, and a single player world.

2. Download a world from the Adventure category into one of the slots.

3. Click Reset world, and upload a single player world.

4. Wait for the loading bar to finish, and then click "Done."

5. Be disappointed.