After respawing the Ender Dragon, while destroying the end crystals, the dragon will eventually despawn mid fight. Non destroyed end crystals stay active and the portal in the center opens so you can leave, but no achievement for killing ender dragon pops. This happened multiple attempts, regardless if the player fighting dies during the fight or not.
Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-31765.
I believe this is caused by the same bug as MCPE-31765.
Your additional information and observations are very useful, so if you can add them to the comments section of the parent issue it would be appreciated.
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