if you log out during the restriction process of the dragon it will freeze and if you brake the restriction crystals (the crystals in the middle) there's a chance that the portal blocks will not appear and you will be stuck unless you die or put more crystals and restart the restriction process and brake them (without logging off) or fight the dragon.
I found this in my survival world after an enderman attacked me and I logged off and it froze and...(everything up there) i then tested it in the newest snapshot and its still there
Thank you for your report!
We're tracking this issue in MC-91636, so this ticket is being resolved and linked as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki