The enderdragon won't spawn on the 18w47b snapshot.
I tried other worlds and the enderdragon won't spawn. I tested it on other worlds and it didn't work so I tried it on older versions and everything worked fine.
I can't downgrade my world so I hope everything will be fixed in the next snapshot.
Related issues
Comments

Thank you for your report!
However, this issue is a Duplicate of MC-137467.
If you have additional information, please add it to that report.
Please search before reporting, as it's likely that one exists already.
Quick Links:
📓 Issue Guidelines – 💬 Community Support – 📧 Customer Support – 📖 Game Wiki