Enderdragon freezes in the middle of a fight and despawns after reopening the world

The enderdragon freezes. Trigger unknown. Happened again after respawning the dragon. When it freezes, the boss health bar disappears and the enderdragon cannot be hit. Reopening the world in this state causes the enderdragon to disappear and the main portal to get filled. The dragon egg and the outer islands portal do not appear. Respawning and killing the dragon does make the outer island portal appear, but the dragon egg does not appear.