Upon reviving the Ender Dragon, standing to near to the End Gateway Portal when it attacks causes it to travel to the Outer End. This creates a new Ender Dragons with full health in the Inner End. Without changing the number of remaining End Crystals to destroy/ been destroyed. Whereas in the Outer End, the first Ender Dragon despawns as soon as you enter the Gateway Portal potentially dealing some damage and knockback.
Thank you for your report!
We're actually already tracking this issue at MCPE-46071, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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