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