Ender Dragon gets stuck on the portal to the Overworld

Ender Dragon gets stuck on the portal to the Overworld

What I expected to happen was...:
It just pass trough the portal to the Overworld

What actually happened was...:
The Dragon stops and start to "fly" on the Portal few seconds with no progress

Steps to Reproduce:
1. Go to the End
2. Wait in creative
3. The Ender Dragon comes after more or less 1 minute and gets stuck on the portal to the Overworld