Leaving the end by creating a portal with eyes of ender and end portal frames causes the dragon to appear where you respawn shortly afterwards. It stays until you re-enter the end where the bedrock portal has the dragon egg and is open.
Edit: After further testing this only happens when the dragon itself goes through the portal.
Thank you for your report!
However, this issue has been closed as a Duplicate of MCPE-31765 due to the more general or descriptive summary.
Your votes, 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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