I began the ender dragon re-spawn in a single player survival world then exited the game before the re-spawn process finished. When I entered the world again the re-spawn process was frozen. I had to use 4 new ender crystals to restart the spawn process so that I could kill the ender dragon and exit the end.
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This actually appears to be a duplicate of MC-91636, which hasn't yet been resolved. If you have any additional information to provide regarding this issue, feel free to comment on the parent ticket. If you feel that this issue is affecting you, feel free to vote for the duplicated ticket, to give Mojang a basic idea of how many people are impacted by the problem. Just to further inform you, this will not determine how quickly the bug gets handled or fixed.
Please provide a screenshot of your game with the F3 debug screen enabled.