When you send a small mob (like a cat) to the End, it will get lost. If it is tamed, you will see a message saying that it suffocated or fell out of the world, even if there are blocks all around the platform.

How to reproduce:

1. Build an end portal
2. Go to the End
3. Place blocks all around the platform if there aren't already.
4. Kill the dragon
5. Return to the Overworld
6. Tame a cat
7. Send the cat through the End Portal
8. Go back through the portal

Observed results:

The cat suffocates or falls in the void

Expected results:

The cat should be on top of the platform

I don't know if this is related, but there are sometimes 2 layers of obsidian where the platform is.